In what sense may the separation of all the individuals of a species into two kinds of individuals, male and female, be called an adaptation? Does any advantage result to the species that would not come from a non-sexual method of reproduction? Many attempts have been made to answer these questions, but with what success I shall now try to show.

There are four principal questions that must be considered:—

I. The different kinds of sexual individuals in the animal and plant kingdoms.

II. The historical question as to the evolution of separate sexes.

III. The factors that determine the sex in each individual developing from an egg.

IV. The question as to whether any advantage is gained by having each new individual produced by the union of two germ-cells, or by having the germ-cells carried by two kinds of individuals.

While our main problem is concerned with the last of these topics, yet there would be little hope of giving a complete answer to it unless we could get some answer to the first three questions.

The Different Kinds of Sexual Individuals

Amongst the unicellular animals and plants the fusion of two (or more) individuals into a single one is generally regarded as the simplest, and possibly also the most primitive, method of sexual reproduction. Two amoebas, or amoeba-like bodies, thus flow together, as it were, to produce a new individual.

In the more highly specialized unicellular animals, the processes are different. Thus in vorticella, a small, active individual unites with a larger fixed individual. The protoplasm fuses into a common mass, and a very complicated series of changes is passed through by the nucleus. In paramoecium, a free-swimming form very much like vorticella, two individuals that are alike unite only temporarily, and after an interchange of nuclear material they separate.

In the lower plants, and more especially in some of the simple aggregates or colonial forms, there are found a number of stages between species in which the uniting individuals are alike, and those in which they are different. There are several species whose individuals appear to be exactly alike; and other species in which the only apparent difference between the individuals that fuse together is one of size; and still other species in which there are larger resting or passive individuals, and smaller active individuals that unite with the larger ones. In several of the higher groups, including the green algÆ and seaweeds, we find similar series, which give evidence of having arisen independently of each other. If we are really justified in arranging the members of these groups in series, beginning with the simpler cases and ending with those showing a complete differentiation into two kinds of germ-cells, we seem to get some light as to the way in which the change has come about. It should not be forgotten, however, that it does not follow because we can arrange such a series without any large gaps in its continuity, that the more complex conditions have been gradually formed in exactly this way from the simplest conditions.

So far we have spoken mainly of those cases in which the forms are unicellular, or of many-celled species in which all the cells of the individual resolve themselves into one or the other kind of germ-cells. This occurs, however, only in the lowest forms. A step higher we find that only a part of the cells of the colony are set aside for purposes of reproduction. The cells surrounding these germ-cells may form distinct organs, which may show certain differences according to whether they contain male or female germ-cells. When these two kinds of cells are produced by two separate individuals, the individuals themselves may be different in other parts of the body, as well as in the reproductive organs.

When this condition is reached, we have individuals that we call males and females, because, although they do not themselves unite to form new individuals, they produce one or the other kind of germ-cell. It is the germ-cells alone that now combine to form the new individual.

Amongst living groups of animals we find no such complete series of forms as exist in plants, and the transition from the one-celled to the many-celled forms is also more abrupt. On the other hand, we find an astonishing variety of ways in which the reproduction is accomplished, and several ways in which the germ-cells are carried by the sexual individuals. Let us examine some of the more typical conditions under the following headings: (1) sexes separate; (2) sexes united in the same individual; (3) parthenogenetic forms; (4) exceptional methods of propagation.

1. Sexes Separate; Unisexual Forms.^[34]—Although the animals with which we are more familiar have the sexes separate, this is far from being universal amongst animals and plants; and, in fact, can scarcely be said to be even the rule. When the sexes are separate they may be externally alike, and this is especially true for those species that do not unite, but set free their eggs and spermatozoa in the water, as fish, frogs, corals, starfish, jellyfish, and many other forms. In other animals there are sometimes other secondary differences in the sexes besides those connected with the organs of reproduction. Such differences are found, as we have seen, in insects, in some spiders, crustaceans, and in many birds and mammals. In a few cases the difference between the sexes is very great, especially when the female is parasitic and the male free, as in some of the crustaceans. In some other cases the male is parasitic on the female. Thus in Bonellia the male is microscopic in size, being in length only one-hundredth part of the female. In Hydatina senta the male is only about a third as large as the female. It has no digestive tract, and lives only a few days. In another rotifer the males are mere sacs enclosing the male reproductive organs.

2. Hermaphroditic Forms.—There are many species of animals and plants in which each individual contains both the male and the female organs of reproduction, and there are whole groups in which only these hermaphroditic forms occur. Thus in the ctenophors the eggs develop along one side of each radial canal and spermatozoa along the other. The group of flatworms is almost exclusively hermaphroditic. The earthworms and the leeches have only these bisexual forms, and in the mollusks, while a few groups have separate sexes, yet certain groups of gasteropods and of bivalve forms are entirely hermaphroditic.

In the common garden snail, although there are two sets of sexual ducts closely united, yet from the same reproductive sac both eggs and sperm are produced. The barnacles and the ascidians are for the most part hermaphroditic forms. Many other examples might be cited, but these will suffice to show that it is by no means unusual in the animal kingdom for the same individual to produce both male and female germ-cells. However, one of the most striking facts in this connection is that self-fertilization seldom takes place, so that the result is the same in certain respects as though separate sexes existed. This point will come up later for further consideration.

3. Parthenogenetic Reproduction.—It has long been known that, in some cases, eggs that are not fertilized will begin to develop and may even produce new individuals. Tichomiroff showed that by rubbing with a brush the unfertilized eggs of the silkworm moth, a larger percentage would produce caterpillars than if they were not rubbed. During the last few years it has been shown that the development of a non-fertilized egg may be started in a number of ways. Such, for example, as by certain solutions of salt or of sugar, by subjecting the eggs to cold, or by simply shaking them.

There are certain groups of animals in which the males appear only at regular (in others at irregular) intervals. In their absence the females produce eggs that develop without being fertilized, i.e. parthenogenetically. The following examples will serve to show some of the principal ways in which this “virgin reproduction” takes place. In the group of rotifers the males are generally smaller than the females and are usually also degenerate. In some species, although degenerate males are present, they are unnecessary, since parthenogenesis is the rule. In still other species no males exist and the eggs develop, therefore, without being fertilized. In some of the lower crustaceans parthenogenesis occurs in varying degrees. In Apus males may be entirely absent at times in certain localities, and at other times a few, or even very many, males may appear. Some species of ostracod crustaceans seem to be purely parthenogenetic; others reproduce by means of fertilized eggs; and others by an alternation of the two processes. The crustaceans of the genus Daphnia produce two kinds of eggs. The summer eggs are small, and have a thin shell. These eggs develop without being fertilized, but in the autumn both male and female individuals develop from these unfertilized eggs, and the eggs of the female, the so-called winter eggs, are fertilized. These are also larger than the summer eggs, have thicker shells, and are much more resistant to unfavorable conditions. They give rise in the following spring to females only, and these are the parthenogenetic individuals that continue to produce during the summer new parthenogenetic eggs.

It is within the group of insects that some of the most remarkable cases of parthenogenesis that we know are found. In the moth, Psyche helix, only females are present, as a rule, but rarely males have been found. In another moth, Solenobia trinquetrella, the female reproduces by parthenogenesis, but at times males appear and may then be even more numerous than the females. In the gall-wasps parthenogenetic generations may alternate with a sexual generation, and it is interesting to note that the sexual and the parthenogenetic generations are so different that they were supposed to belong to separate species, until it was found that they were only alternate generations of the same species.

The aphids or plant-lice reproduce during the summer by parthenogenesis, but in the autumn winged males and females appear, and fertilized winter eggs are produced. From these eggs there develop, in the following spring, the wingless parthenogenetic summer forms, which produce the successive generations of the wingless forms. As many as fourteen summer broods may be produced. By keeping the aphids in a warm temperature and supplying them with plenty of moist food, it has been possible to continue the parthenogenetic reproduction of the wingless forms for years. As many as fifty successive broods have been produced in this way. It has not been entirely determined whether it is the temperature or a change in the amount, or kind, of food that causes the appearance of the winged males and females, although it seems fairly certain that diminution in the food, or in the amount of water contained in it, is the chief cause of the change.

In the honey-bee the remarkable fact has been well established that fertilized eggs give rise only to females (queens and workers), while unfertilized eggs develop into males. Whether a fertilized egg becomes a queen or a worker (sterile female) depends solely on the kind of food that is given to the young larva, and this is determined, in a sense, entirely by the bees themselves.

In plants also there are many cases of parthenogenesis known. Some species of Chara when kept under certain conditions produce only female organs, and seem to produce new plants parthenogenetically. In this case it appears that the same conditions that caused the plants to produce only female organs may also lead to the development of the egg-cells without fertilization. In fact it is only by a combination of this kind that parthenogenesis could arise. The result is similar when the eggs of insects produce only females whose eggs are capable of parthenogenetic development. If a case should arise in which only females appeared whose eggs did not possess the power of parthenogenetic development, the species would die out.

In the green alga, Spirogyra, it has been found that if conjugation of two cells is prevented, a single cell may become a parthenogenetic cell. In a number of parasitic fungi the male organs appear to be degenerate, and from the female organs parthenogenetic development takes place. A small number of flowering plants are also capable of parthenogenetic reproduction.

There is a peculiarity in the development of the parthenogenetic eggs of animals that will be more fully discussed later, but may be mentioned here. Ordinarily an egg that becomes fertilized gives off two polar bodies, but in a number of cases in which parthenogenetic development occurs it has been found that only one polar body is given off. It is supposed that in such cases one polar body is retained, and that it plays the same part as the entrance of the spermatozoon of the male.

4. Exceptional Cases.—Occasionally in a species that is unisexual an individual is found that is bisexual. The male of the toad, Pelobates fuscus, has frequently a rudimentary ovary in front of the testis. The same thing has been found in several species of fish. In Serranus, a testis is present in the wall of the ovary, and the eggs are said to be fertilized by the spermatozoa of the same individual. In frogs it has been occasionally found that ovary and testis may be associated in the same individual, or a testis may be present on one side, and a testis with an anterior ovarian portion on the other. Cases like these lead up to those in which the body itself may also show a mosaic of sex-characters, and it is noticeable that when this occurs there is nearly always a change in the reproductive organs also. Thus butterflies have been found with the wings and the body of one side colored like the male and the other side like the female. Similar cases have also been found in bees and ants. Bees have been found with the anterior part of the body of one sex and posterior part of another!

The preceding cases illustrate, in different ways, the fact that in the same individual both kinds of reproductive organs may suddenly appear, although it is the rule in such species that only one set develops. Conversely, there are cases known, especially amongst plants, in which individuals, that usually produce male and female organs (or more strictly spores of two kinds from which these organs develop), produce under special conditions only one or the other kind. Facts like these have led to the belief that each individual is potentially bisexual, but in all unisexual forms one sex predominates, and the other remains latent. This idea has been the starting-point for nearly all modern theories of sex.

An excellent illustration of this theory is found in those cases in which the same individual may be male at one time and female at another. For instance, it is said that in one of the species of starfish (Asterina gibbosa) the individuals at Roscoff are males for one or two years, and then become females. At Banyuls they are males for the first two or three years, and then become females; while at Naples some are always males, others females, some hermaphrodites, others transitional as in the cases just given. In one of the isopod crustaceans, Angiostomum, the young individuals are males and the older females. In Myzostomum glabrum the young animal is at first hermaphroditic, then there is a functional male condition, followed by a hermaphroditic condition, and finally a functional female phase, during which the male reproductive organs disappear.

The flowers of most of the flowering plants have both stamens and pistils, which contain the two kinds of spores out of which the male and female germ-cells are formed. The stamens become mature before the pistils, as a rule, but in some cases the reverse is the case. This difference in the time of ripening of the two organs is often spoken of as an adaptation which prevents self-fertilization. The latter is supposed to be less advantageous than cross-fertilization. This question will be more fully considered later.

Before we come to an examination of the question of the adaptations involved in the cases in which the sexes are separate, and the different times at which the sex-cells are ripened, it will be profitable first to examine the question as to what determines in the egg or young whether a male or a female or a hermaphroditic form shall arise.

The Determination of Sex

A large number of views have been advanced as to what determines whether an egg will give rise to a male or to a female individual. The central question is whether the fertilized egg has its sex already determined, or whether it is indifferent; and if the latter, what external factor or factors determine the sex of the embryo. Let us first examine the view that some external factor determines the sex of the individual, and then the evidence pointing in the opposite direction. Among the different causes suggested as determining the sex of the embryo, that of the condition of the egg itself at the time of fertilization has been imagined to be an important factor in the result. Another similar view holds that the condition of the spermatozoon plays the same rÔle. For instance, it has been suggested that if the egg is fertilized soon after it leaves the ovary, it produces a female, but if the fertilization is delayed, a male is produced. It has also been suggested that the relative age of the male and the female parents produces an effect in determining the sex of the young. There is no satisfactory evidence, however, showing that this is really the case.

Another view suggested is that the sex is determined by the more vigorous parent; but again there is no proof that this is the case, and it would be a difficult point to establish, since as Geddes and Thompson point out, what is meant by greater vigor is capable of many interpretations. Somewhat similar is the idea that if the conditions are favorable, the embryo develops further, as it were, and becomes a male; but there are several facts indicating that this view is untenable.

DÜsing maintains that several of these factors may play a part in determining the sex of the embryo, and if this be true, the problem becomes a very complex one. He also suggests that there are self-regulative influences of such a kind that, when one sex becomes less numerous, the conditions imposed in consequence on the other sex are such as to bring the number back to the normal condition; but this idea is far from being established. The fact that in some species there are generally more individuals of one sex than of the other shows that this balance is not equally adjusted in such forms.

Of far greater value than these speculations as to the origin of sex are the experiments that appear to show that nutrition is an important factor in determining sex. Some of the earlier experiments in this direction are those of Born and of Yung. By feeding one set of tadpoles with beef, Yung found the percentage of females that developed to be greatly increased, and a similar increase was observed when the tadpoles were fed on the flesh of fish. An even greater effect was produced by using the flesh of frogs, the percentage rising to 92 females in every hundred. These results have been given a different interpretation by PflÜger and by others, and, as will be pointed out later, there is a possible source of error that may invalidate them.

Somewhat similar results have been obtained by Nussbaum for one of the rotifers. He found that if the rotifer is abundantly fed in early life, it produces female eggs, that is, larger eggs that become females; while if sparingly fed, it produces only small eggs, from which males develop. It has been claimed also in mammals, and even in man, that sex is to some extent determined by the nourishment of the individual.

Some experiments made by Mrs. Treat with caterpillars seemed to show that if the caterpillars were well nourished more female moths were produced, and if starved before pupation more males emerged. But Riley has pointed out that since the larger female caterpillars require more food they will starve sooner than the males, and, in consequence, it may appear that proportionately more male butterflies are born when the caterpillars are subjected to a starvation diet. This point of view is important in putting us on our guard against hastily supposing that food may directly determine sex. Unless the entire number of individuals present at the beginning of the experiment is taken into account, the results may be misleading, because the conditions may be more fatal to one sex than to the other.

In some of the hymenopterous insects, the bees for example, it has been discovered that the sex of the embryo is determined by the entrance, or lack of entrance, of the spermatozoon. In the honey-bee all the fertilized eggs produce females and the unfertilized eggs males. The same relation is probably true also in the case of ants and of wasps. In the saw-flies, the conditions are very remarkable. Sharp gives the following account of some of these forms:^[35]—“It is a rule in this family that males are very much less numerous than females, and there are some species in which no males have been discovered. This would not be of itself evidence of the occurrence of parthenogenesis, but this has been placed beyond doubt by taking females bred in confinement, obtaining unfertilized eggs from them, and rearing the larvÆ produced from the eggs. This has been done by numerous observers with curious results. In many cases the parthenogenetic progeny, or a portion of it, dies without attaining full maturity. This may or may not be due to constitutional weakness, arising from the parthenogenetic state. Cameron, who has made extensive observations on this subject, thinks that the parthenogenesis does involve constitutional weakness, fewer of the parthenogenetic young reaching maturity. This, he suggests, may be compensated for—when the parthenogenetic progeny is all of the female sex—by the fact that all those that grow up are producers of eggs. In many cases the parthenogenetic young of TenthredinidÆ are of the male sex, and sometimes the abnormal progeny is of both sexes. In the case of one species—the common currant-fly, Nematus ribesii—the parthenogenetic progeny is nearly, but not quite always, entirely of the male sex; this has been ascertained again and again, and it is impossible to suggest in these cases any advantage to the species to compensate for constitutional parthenogenetic weakness. On the whole, it appears most probable that the parthenogenesis, and the special sex produced by it, whether male or female, are due to physiological conditions of which we know little, and that the species continues in spite of the parthenogenesis rather than profits by it. It is worthy of remark that one of the species in which parthenogenesis with the production of males occurs—Nematus ribesii—is perhaps the most abundant of saw-flies.”

It has been pointed out that in a number of species of animals and plants only parthenogenetic females are present at certain times. In a sense this means a preponderance of one sex, but since the eggs are adapted only to this kind of development, it may be claimed that the conditions in such cases are somewhat different from those in which eggs that would be normally fertilized may develop in the absence of fertilization. Nevertheless, it is generally supposed that the actual state of affairs is about the same. It is usually assumed, and no doubt with much probability, that these parthenogenetic forms have evolved from a group which originally had both male and female forms. One of the most striking facts in this connection is that in the groups to which these parthenogenetic species belong there are, as a rule, other species with occasional parthenogenesis, and in some of these the males are also fewer in number than the females.

In the aphids, the parthenogenetic eggs give rise during the summer to parthenogenetic females, but in the autumn the parthenogenetic eggs give rise without fertilization both to males and to females. It appears, therefore, that we can form no general rule as to a relation between fertilization and the determination of sex. While in certain cases, as in the bees, there appears to be a direct connection between these two, in other cases, as in that of the aphids just mentioned, there is no such relation apparent.

Geddes and Thompson have advocated a view in regard to sex which at best can only serve as a sort of analogy under which the two forms of sex may be considered, rather than as a legitimate explanation of the phenomenon of sex. They rest their view on the idea that living material is continually breaking down and building up. An animal in which there is an excess of the breaking-down process is a male, and one that is more constructive is a female. Furthermore, whichever process is in the excess during development determines the sex of the individual. Thus, if conditions are very favorable, there will be more females produced; but if, on the other hand, there is an excess of the breaking-down process, males are produced. So far, the process is conceived as a purely physiological one, but to this the authors then apply the selection hypothesis, which, they suppose, acts as a sort of break or regulation of the physiological processes, or in other words as a directive agent. They state: “Yet the sexual dimorphism, in the main, and in detail, has an adaptive significance, also securing the advantages of cross-fertilization and the like, and is, therefore, to some extent the result of the continual action of natural selection, though this may, of course, check variation in one form as well as favor it in another.” Disregarding this last addition, with which Geddes and Thompson think it necessary to burden their theory, let us return to the physiological side of the hypothesis. Their idea appears to me a sort of symbolism rather than a scientific attempt to explain sex. If their view had a real value, it ought to be possible to determine the sex of the developing organism with precision by regulating the conditions of its growth, and yet we cannot do this, nor do the authors make any claim of being able to do so. The hypothesis lacks the only support that can give it scientific standing, the proof of experiment.

There have been made, from time to time, a number of attempts to show that the sex of the embryo is predetermined in the egg, and is not determined later by external circumstances. In recent years this view has come more to the front, despite the apparent experimental evidence which seemed in one or two cases to point to the opposite view. One of the most complete analyses of the question is that of CuÉnot, who has attempted to show that the sex of the embryo is determined in the egg, before or at the time of fertilization. He has also examined critically the evidence that appeared to show that external conditions, acting on the embryo, may determine the sex, and has pointed out some possible sources of error that had been overlooked. The best-known case is that of the tadpole of the frog, but CuÉnot shows not only that there are chances of error in this experiment as carried out, but also, by his own experiments and observations, that the facts themselves are not above suspicion. He points out that at the age at which some of the tadpoles were when the examination was made, it was not always possible to tell definitely the sex of the individual, and least of all by means of the size alone of the reproductive organs, as was supposed, in one case at least, to be sufficient. In his own experiments he did not find an excess of one sex over the other as a result of feeding.

CuÉnot points out that Brocadello found that the larger eggs laid by the silkworm give rise to from 88 to 95 per cent of females, and the small eggs to from 88 to 92 per cent of males. Joseph has confirmed this for Ocneria dispar, and CuÉnot himself also reached this conclusion. Korschelt found that the large eggs of Dinophilus produced females and the small ones males. CuÉnot experimented with three species of flies, and found that when the maggots were well nourished the number of the individuals of the two sexes was about equal, and when poorly nourished there were a few more females in two cases, and in another about the same number of males and females.

It has been claimed that the condition of nourishment of the mother may determine the number of eggs of a particular sex, but CuÉnot found, in three species of flies which he raised, that there was a slight response in the opposite direction. He concludes that the condition of the mother is not a factor in the determination of sex.

The first egg of the two laid in each set by the pigeon is said, as a rule, to produce a male, and the second a female. Both Flourens and CuÉnot found this to be the case in the few instances that they examined, but CuÉnot has shown that this does not always happen. Even when this occurs, it has not been determined whether the result depends on something in the egg itself, that causes a male egg to be set free first, or on some external condition that determines that the first egg shall become a male. It has been claimed that the age of the spermatozoon might in this and in other cases determine the result; but Gerbe has shown that if the domestic hen is isolated for fifteen days after union with the male, she will continue to produce fertile eggs from which both sexes are produced, without showing any relation between the time the eggs are laid and the particular sex that develops.

CuÉnot does not discuss whether sex is determined by the nucleus or by the protoplasm, but if, as he thinks probable, the size of the egg is a determining factor, it would appear that the protoplasm must be the chief agent. Even if this were the case it would still be possible that the size of the egg itself might be connected with some action on the part of the nucleus. If, as seems probable, identical twins come from halves of the same egg, then, since they are of the same sex, the absolute amount of protoplasm cannot be a factor in sex determination.

As a basis for the discussion that follows, certain processes that take place during the maturation divisions of the egg and of the spermatozoon must be briefly noticed. After the egg leaves the ovary it extrudes a minute body called the first polar body (Fig. 6 B, C, D). This process of extrusion is really a cell division accompanied by the regular mitotic division of the nucleus; but since one of the products of the division, the polar body, is extremely small, the meaning of the process was not at first understood. The half of the nucleus, that remains in the egg, divides again, and one of its halves is thrown out into a second polar body (Fig. 6 E, F, G)). Meanwhile, the first polar body has divided into two equal parts, so that we find now three polar bodies and the egg (Fig. 6 G)). A strictly analogous process takes place in the formation of the spermatozoa (Fig. 7 B-F). The mother-cell of the spermatozoon divides into two parts, which are equal in this case (Fig. 7 B-D). Each of these then divides again (Fig. 7 E, F), producing four cells that are comparable to the three polar bodies and the mature egg. Each of the four becomes a functional spermatozoon (Fig. 7 G, H). Thus while in the maturation of the egg only the egg itself is capable of development, in the case of the male cells all four products of the two maturation divisions are functional.

Now, in certain cases of parthenogenesis, it has been found that one of the polar bodies may not be given off, but, remaining in the egg, its nucleus reunites with the egg nucleus, and thus takes the place of the spermatozoon, which does exactly the same thing when it fertilizes the egg, i.e. the nucleus of the spermatozoon unites with the nucleus of the egg. This fact in regard to the action of the polar body in fertilization is not as surprising as appears at first sight, for if each of the polar bodies is equivalent to a spermatozoon, the fertilization of the egg by one of its own polar bodies conforms to theory.

There is a considerable body of evidence showing that in many eggs at one of the two maturation divisions the chromatin rods derived from the nucleus are divided crosswise (Fig. 6 B, C). The same thing occurs at one of the two divisions in the formation of the spermatozoon (Fig. 7 B, C). At the other division to form the other polar body (or the other sperm-cell) the chromatin rods appear to be split lengthwise, as in ordinary cell division (Fig. 6 E, F, G). In recent years the cross-division of the chromatin rods has attracted a great deal of notice, and Weismann in particular drew attention to the possible importance of this kind of division.

There is another fact that gives this division especial significance. It has been discovered that the number of chromosomes that appears in each dividing cell of the organism is a constant number, but it has also been discovered that the egg, before extruding its polar bodies, and the mother-cell of the spermatozoon (Figs. 6, 7 B), contain exactly half of the number of chromosomes that are characteristic of the body-cells of the same animal (Figs. 6, 7 A). Now there is good evidence to show that the reduction in number is due to the chromosomes uniting sometimes end to end in pairs, as shown in Figures A and B. Furthermore, it has been suggested that at one of the maturation divisions, when the chromosomes divide crosswise, the united chromosomes are separated (Figs. 6, 7 B, C), so that one remains in the egg and the other goes out into the polar body. The same thing is supposed to occur at one of the maturation divisions of the sperm mother-cell. A further consideration of capital importance in this connection has been advocated by Montgomery and by Sutton, namely, that, when the chromosomes unite in pairs, a chromosome from one parent unites with one from the other parent. Consequently at one of the two reduction divisions maternal and paternal chromosomes may separate again, some to go to one cell, some to the other.

When the spermatozoon enters the egg it brings into the egg as many new chromosomes as the egg itself possesses at this time, and the two nuclei, uniting into a single one, furnish the total number of chromosomes characteristic of the animal that develops from the egg. At first the chromosomes that are brought in by the spermatozoon lie at one side of the fused nucleus, and those from the egg itself at the other side. This arrangement appears, however, in some cases at least, to be lost later. At every division of the nucleus, each chromosome divides and sends a half to each of the daughter-nuclei. Thus every cell in the body contains as many paternal as maternal chromosomes. This statement also applies to the first cells that go into the reproductive organs, some of which become the mother-cells of the germ-cells. Later, however, in the history of the germ-cells,—just before the maturation divisions,—these chromosomes are supposed to unite in pairs, end to end, as explained above, to give the reduced number. Later there follows the separation of these paired chromosomes at one of the two maturation divisions. If at this time all the paternal chromosomes should pass to one pole, and all the maternal to the other, the germ-cell ceases to be mixed, and becomes purely paternal or maternal. If this ever occurs, the problem of heredity may become simplified, and even the question of sex may be indirectly involved; but it has not been established that, when the reduced number of chromosomes is formed, there is a strict union between the paternal and maternal chromosomes, and if not, the subsequent separation is probably not along these lines. If, however, the chromosomes contain different qualities, as Boveri believes, there may be two kinds of eggs, and two kinds of spermatozoa in regard to each particular character. It is this last assumption only that is made in Mendel’s theory of the purity of the germ-cells.

Several attempts have been made at different times to connect the facts in regard to the extrusion of the polar bodies with those involved in the determination of sex. Minot suggested, in 1877, that the egg ejects by means of the polar bodies its male elements, which are again received in the fertilization of the egg by the spermatozoon. The same idea has also been expressed by others. It has been objected to this view that one polar body ought to suffice, and that no similar throwing out of part of its substance is found in the process of formation of the spermatozoon, which should, on the hypothesis, throw out its female elements. It would seem, on first thought, that this view might find support in the idea expressed above, namely, that in one of the polar bodies half of the chromosomes pass out, so that there is conceivably a separation of the maternal from the paternal. If this were the case also in the spermatozoa, then two of each four would be paternal and two maternal. This is, however, a very different thing from supposing them to be male and female, for it by no means follows, because the chromosomes correspond to those of the father or of the mother in the sum of their characters, that they are, therefore, also male or female in regard to sex.

It has been pointed out already, that in most parthenogenetic eggs only one polar body is extruded. There are, it is true, a few apparent exceptions to this rule, but in most cases it is certain that only one is extruded. In several cases the beginning of the formation of the second maturation division of the nucleus takes place, but after the chromosomes have divided they come together again in the nucleus. If each polar body be interpreted as equivalent to a spermatozoon, then this result is rather a process of self-fertilization than true parthenogenesis. It is, nevertheless, true that in some cases development seems to go on after both polar bodies have been extruded. Moreover, it has been found possible to cause the eggs of the sea-urchin to begin their development by artificial solutions after they have extruded both polar bodies. A single spermatozoon may also produce an embryo if it enters a piece of egg-protoplasm without a nucleus. The last instance is a case of male parthenogenesis, and if the theory of the equivalency of spermatozoon and egg be correct, this is what should occur.

Quite recently, CuÉnot, Beard, Castle, and Lenhossek have contended that the differentiation of sex is the outcome of internal factors. They think that the view that sex is determined by external agents is fundamentally erroneous. The fallacies that have given rise to this conception, Castle points out, are, first, that in animals that reproduce sometimes by parthenogenesis and sometimes by fertilized eggs, the former process is favored by good nutrition and the latter by poor nutrition. This only means, in reality, Castle thinks, that parthenogenetic reproduction is favored by external conditions, and this kind of reproduction, he thinks, is a thing sui generis, and not to be compared to the formation of more females in the sexual forms of reproduction. There is no proof, however, that this is anything more than a superficial distinction, and it ignores the fact that in ordinary cases the females sometimes lay parthenogenetic eggs which differ, as far as we can see, from eggs that are destined to be fertilized in no important respect. More significant, it seems to me, is the fact that only parthenogenetic females develop the following spring from the fertilized eggs of the last generation of the autumn series, whose origin is described to be due to lack of food. We find, in the case of aphids, that unfertilized parthenogenetic eggs and also fertilized eggs give rise to females only, while a change in the amount of food causes the parthenogenetic eggs to give rise both to males and to females. This point is not, I think, fully met by Castle, for even if the change in food does not, as he claims, cause only one sex to appear, yet lack of food does seem to account for the appearance of the males at least.

The other fallacy, mentioned by CuÉnot, is that the excess of males that has been observed when the food supply is limited is due to the early death of a larger percentage of females, which require more food, but this still fails to account for the excess of females when more food is given, provided Yung’s experiments on tadpoles are correct. It may be, however, in the light of PflÜger’s results, that there has been some mistake in the experiments themselves.

We may now proceed to examine Castle’s argument, attempting to show in what way sex is predetermined in the embryo. His hypothesis rests on the three following premises: “(1) the idea of Darwin, that in animals and plants of either sex the characters of the opposite sex are latent; (2) the idea of Mendel, that in the formation of the gametes [germ-cells] of hybrids a segregation of the parental characters takes place, and when in fertilization different segregated characters meet, one will dominate, the other become latent or recessive; (3) the idea of Weismann, that in the maturation of egg and spermatozoon a segregation is attended by a visible reduction in the number of chromosomes in the germinal nuclei.”

Expressed in a somewhat more general way, Castle suggests that each egg and each spermatozoon is either a male or a female germ-cell (and not a mixture of the two), and when a female egg is fertilized by a male spermatozoon, or vice versa, the individual is a sexual hybrid with one sex dominating and the other latent. The assumption that there are two kinds of eggs, male and female, and two kinds of spermatozoa, male and female, is not supported by any direct or experimental evidence. Moreover, in order to carry out the hypothesis, it is necessary to make the further assumption that a female egg can only be fertilized by a male spermatozoon, and a male egg by a female spermatozoon. While such a view is contrary to all our previous ideas, yet it must be admitted that there are no facts which disprove directly that such a selection on the part of the germ-cells takes place. If these two suppositions be granted, then Castle’s hypothesis is as follows:—

In order that half of the individuals shall become males and half females it is necessary to assume that in some individuals the male element dominates and in others the female, and since each fertilized egg contains both male and female elements, it is necessary to assume that either the egg or the spermatozoon contains the dominating element.

Castle supposes that in hermaphroditic organisms the two characters “exist in the balanced relationship in which they were received from the parents,” but, as has just been stated, in unisexual forms one or the other sex dominates, except of course in those rare cases, as in the bees and ants, where half of the body may bear the characters of one sex, and the other half that of the other sex.

In parthenogenetic species the female character is supposed to be uniformly stronger, so that it dominates in every contest, “for the fertilized egg in such species develops invariably into a female.” Under certain circumstances, as Castle points out, the parthenogenetic female produces both males and females, and this is also true in the occasional development of the unfertilized egg of the silkworm moth, and of the gypsy moth, in which both male and female individuals are produced by parthenogenesis. These facts show that even in unfertilized eggs both sexes are potentially present; but this might be interpreted to mean that some eggs are male and some female, rather than that each egg has the possibility of both kinds of development. If, however, one polar body is retained in these parthenogenetic eggs, then ex hypothese each egg would contain the potentialities of both sexes (if the polar body were of the opposite sex character). It seems necessary to make this assumption because in some parthenogenetic forms males and females may be produced later by each individual, as in the aphids, and this could not occur if we assume that some parthenogenetic eggs are purely male and some female.

Castle assumes, in fact, that in animals like daphnids and rotifers one polar body only is extruded, and the other (the second) is retained in the egg, and hence the potentiality of producing males is present. In the honey-bee, on the contrary, Castle assumes that both polar bodies are extruded in the unfertilized egg (and there are some observations that support this idea), and since only males are produced from these, he believes it is the female element that has been sent out into the second polar body. This hypothesis is necessary, because Castle assumes that when both elements are present in the bee’s eggs, the female element dominates. “Hence, if the egg which has formed two polar cells develops without fertilization, it must develop into a male. But if such an egg is fertilized, it invariably forms a parthenogenetic female ? (?), that is, an individual in which the male character is recessive. Accordingly the functional spermatozoon must in such cases invariably bear the female character, and this is invariably dominant over the male character when the two meet in fertilization.”

If it should prove generally true that the size of the egg is one of the factors determining the sex, we have still the further question to consider as to whether some eggs are bigger because they are already female, or whether all eggs that go beyond a certain size are females, and all those that fail to reach this are males. If this is the case, an animal might produce more females if the external conditions were favorable to the growth of the eggs, and if in some cases these large eggs were capable of developing, parthenogenetic races might become established. Should, however, the conditions for nutrition become less favorable, some of the eggs might fall below the former size and produce males. It is not apparent, however, why all the fertilized autumn eggs of the aphids should give rise to females, for although these eggs are known to be larger than the summer eggs, yet they are produced under unfavorable conditions.

The preceding discussion will show how far we still are from knowing what factors determine sex. Castle’s argument well illustrates how many assumptions must be made in order to make possible the view that sex is a predetermined quality of each germ-cell. Even if these assumptions were admissible, we still return to the old idea that the fertilized egg has both possibilities, and something determines which shall dominate. Until we have ascertained definitely by experimental work whether the sex in some forms can be determined by external conditions, it is almost worthless to speculate further. Whatever decision is reached, the conclusion will have an immediate bearing on the question to be next discussed. Meanwhile, we can at least examine some of the theories that have been advanced as to what advantage, if any, has been gained by having the individuals of many classes divided into two kinds, male and female.

Sex as a Phenomenon of Adaptation

Of what advantage is it to have the individuals of many species separated into males and females? It is obviously a disadvantage from the point of view of propagation to have half of the individuals incapable of producing young, and the other half also incapable of doing so, as a rule, unless the eggs are fertilized by the other sex. Is there any compensation gained because each new individual arises from two parents instead of from one? Many answers have been attempted to these questions.

At the outset it should be recognized that we are by no means forced to assume, as is so often done, that because there is this separation of the sexes it must have arisen on account of its advantage to the species. Whether the result may be of some benefit regardless of how it arose, may be an entirely different question. It would be extremely difficult to weigh the relative advantages (if there are any) and disadvantages (that are obvious as pointed out above), nor is it probable that in this way we can hope to get a final answer to our problem. We may begin by examining some of the modern hypotheses that have been advanced in this connection.

Darwin has brought together a large number of facts which appear to show the beneficial effects of the union of germ-cells from two different individuals. Conversely, it is very generally believed, both by breeders and by some experimenters, that self-fertilization in the case of hermaphroditic forms leads, in many cases, though apparently not in all, to the production of less vigorous offspring. Darwin’s general position is that it is an advantage to the offspring to have been derived from two parents rather than to have come from the union of the germ-cells of the same individual, and he sees, in the manifold contrivances in hermaphroditic animals and plants to insure cross-fertilization, an adaptation for this purpose.

This question of whether self-fertilization is less advantageous than cross-fertilization is, however, a different question from that of whether non-sexual methods of reproduction are less advantageous than sexual ones. Since some plants, like the banana, have been propagated for a very long time solely by non-sexual methods without any obvious detriment to them, it is at first sight not easy to see what other advantage could be gained by the sexual method. The case of the banana shows that some forms do not require a sexual method of propagation. Other forms, however, are so constituted, as we find them, that they cannot reproduce at the present time except by the sexual method. In other words, the latter are now adapted, as it were, to the sexual method, and there is no longer any choice between the two methods. The question of whether a non-sexual form might do better if it had another method of propagation is not, perhaps, a profitable question to discuss.

What we really need to know is whether or not the sexual method was once acquired, because it was an advantage to a particular organism, or to the species to reproduce in this way. It is assumed by many writers that this was the case, but whether they have sufficient ground for forming such an opinion is our chief concern here. On the other hand, it is conceivable, at least, that if the sexual method once became established, it might continue without respect to any superiority it gave over other methods, and might finally become a necessary condition for the propagation of particular species. Thus the method would become essential to propagation without respect to whether the species lost more than it gained. Whichever way the balance should turn, it might make little difference, so long as the species was still able to propagate itself.

Brooks made the interesting and ingenious suggestion that the separation of the sexes has been brought about as a sort of specialization of the individuals in two directions. The male cells are supposed to accumulate the newly acquired characters, and represent, therefore, the progressive element in evolution. The female cells are the conservative element, holding on to what has been gained in the past. It does not seem probable, in the light of more recent work, that this is the function of the two sexes, and it is unlikely that we could account for the origin of the two sexes through the supposed advantage that such a specialization might bring about. A number of writers, Galton, Van Beneden, BÜtschli, Maupas, and others, have looked at the process of sexual reproduction as a sort of renewal of youth, or rejuvenescence of the individuals. There is certainly a good deal in the process to suggest that something of this sort takes place, although we must be on our guard against assuming that the rejuvenescence is anything more than the fulfilment of a necessary stage in the life history. Weismann has ridiculed this suggestion on the ground that it is inconceivable that two organisms, decrepit with old age, could renew their youth by uniting. Two spent rockets, he says, cannot be imagined to form a new one by combining. There is apparent soundness in this argument, if the implication is taken in a narrow physical sense. If, on the other hand, the egg is so constituted that at a certain stage in its development an outside change is required to introduce a new phase, then the conception of rejuvenescence does not appear in quite so absurd a light.

This hypothesis of rejuvenescence is based mainly on certain processes that take place in the life history of some of the unicellular animals. Let us now see what this evidence is. The results of certain experiments carried out by Maupas on some of the ciliate protozoans have been fruitful in arousing discussion as to the ultimate meaning of the sexual process. Maupas’ experiments consisted in isolating single individuals, and in following the history of the descendants that were produced non-sexually by division. He found that the descendants of an individual kept on dividing, but showed no tendency to unite with each other. After a large number of generations had been passed through (in Stylonychia pustulata, between 128 and 175; in Leucophys patula, 300 to 450; and in Onychodromus grandis, 140 to 230 generations), the division began to slow down, and finally came to a standstill. Maupas found that if he took one of these run-down individuals, and placed it with another in the same condition from another culture, that had had a different parentage, the two would unite and the so-called process of conjugation take place. This process consists for the species used, in the temporary union and partial fusion of the protoplasm of the two individuals, of an interchange of micronuclei, and of a fusion, in each individual, of the micronucleus received from the other individual with one of its own. The individuals then separate, and a new nucleus (or nuclei) is formed out of the fused pair.

The individuals in question, in which this interchange of micronuclei has taken place, undergo a change, and behave differently from what they did before. They feed, become larger and less vacuolated, and are more active. They soon begin once more to divide. Maupas found that an individual that has conjugated will run through a new cycle of divisions, which will, however, after a time also slow down, unless conjugation with another individual having a different history takes place. If conjugation is prevented, the individual will die after a time. These results seemed to show that the division phase of the life history cannot go on indefinitely, and that through conjugation the individual is again brought back to the starting-point.

Quite recently Calkins has carried out a somewhat similar series of experiments, which have an important bearing on the interpretation of Maupas’ results. The experiment of isolating an individual and tracing the career of its descendants was repeated with the following results: two series were started, the original forms coming from different localities. Of their eight descendants four of each were isolated. The remaining four of each set were kept together as stock material. The rate of division was taken as the measure of vitality. The animals divided more or less regularly from February to July. After each division (or sometimes after two divisions) the individuals were separated. About the 30th of July the paramoecia began to die “at an alarming rate, indicating that a period of depression had apparently set in, or degeneration in Maupas’ sense.” Up to this time the animals had been living in hay infusion, renewed every few days, from which they obtained the bacteria on which they feed. Calkins tried the effect of putting the weakened paramoecia into a new environment. Infusion of vegetables gave no good results, but meat infusions proved successful. “The first experiment with the latter was with teased liver, which was added to the usual hay infusion. The result was very gratifying, for the organisms began immediately to grow and to divide, the rate of division rising from five to nine divisions in successive ten-day periods.” This beneficial effect was not lasting, however, and after ten days the paramoecia began to die off faster than before, and the renewed application of the liver extract failed to revive them. A number of other extracts were then tried without effect. Finally they were transferred to the clear extract of lean beef in tap water. The effect of this medium was interesting, for, although it restored the weakened vitality, there was no rapid increase in the rate of division, as when first treated with the teased liver. The infusoria were, however, now large and vigorous, and did not die unless transferred from the beef medium to the usual hay infusion. “When this was attempted, they would become abnormally active and would finally die. The division rate gradually increased during the month of August until, in the last ten days, they averaged six generations. Finally, in September, the attempts to get them back on the old diet of hay infusion were successful, and then the division rate went up at once to twelve times in ten days, and a month later they were dividing at the rate of fifty times a month.”

“These cultures went on well until December, when the paramoecia began to die again. They were saved once more with the beef extract, and when returned later to the hay infusion continued through another cycle of almost three months. Some of these were treated, once a week for twenty-four hours, with the beef extract, and while the two sets ran a parallel course at first, those kept continuously in the hay infusion died after a time, but those that had been put once a week into the beef extract (which had been stopped, however, in March) continued their high rate of division throughout the period of decline of their sister cells, and did not show signs of diminished vitality until the first period in June.” At this time their rate of division increased rapidly. They were put back into the beef extract, but it failed now to have a beneficial effect, and the animals continued to die at a rapid rate. To judge from the appearance of the organisms the new decline was due to a different cause; for, while in the former periods the food vacuoles contained undigested food, at this period the interior was free from food masses. The protoplasm became granular and different from that of a healthy individual. None of the former remedies were now of any avail. “When the last of the B-series stock had died in the five hundred and seventieth generation (June 16th), it looked as though the cultures were about to come to an end.” Extract of the brain and of the pancreas were then tried. To this a favorable response took place at once. The organisms became normal in appearance and began to divide. After forty-eight hours’ treatment they were returned to the usual hay infusion. Here they continued to multiply and reached on June 28th the six hundred and sixty-fifth generation.

There can be no doubt that the periods of depression that appear in these infusoria kept in cultures can be successfully passed if the animals are introduced into a new environment. Without a change of this sort they will die. Calkins thinks that the effect is produced, not by the new kind of food that is supplied, but by the presence of certain chemical compounds. The beef extract “does not have a direct stimulating effect upon the digestive process and upon division, for, while the organisms are immersed in it, there is a very slow division rate; when transferred again to the hay infusion, however, they divide more rapidly than before.”

This brings us back to the idea of the “renewal of youth” through conjugation. Maupas claimed that union of individuals having the same immediate descent is profitless. Calkins suggests that this is due to the similarity in the chemical composition of the protoplasm of the two individuals. When in nature two individuals that have lived under somewhat different conditions conjugate, the result should be beneficial, since there takes place the commingling of different protoplasms.

Calkins’s work has shown that by means of certain substances much the same effect can be produced as that which is supposed to follow from the conjugation of two unrelated individuals. The presumption, therefore, is in favor of the view that the two results may be brought about in the same way, although we should be careful against a too ready acceptation of this plausible argument; for we have ample evidence to show that many closely similar (if not identical) responses of organisms may be brought about by very different agencies. The experiments seem to indicate that paramoecium might go on indefinitely reproducing by division, provided its environment is changed from time to time. If this is true, it is conceivable that the same thing is accomplished through conjugation. In the light of this possible interpretation much of the mystery connected with the term rejuvenescence is removed, for we see that there is nothing in the process itself except that it brings the organism into a new relation with other substances. Difficult as it assuredly is to understand how this benefits the animal, the experimental fact shows, nevertheless, that such a change is for its good. That there is really nothing in the process of conjugation itself apart from the difference in the constitution of the conjugating individuals is shown by the result that the union of individuals having the same history and kept under the same conditions is of no benefit.

Can we apply this same conception to the process of fertilization in the higher animals and plants? Is the substance of which their bodies are made of such a sort that it cannot go on living indefinitely under the same conditions, but must at times be supplied with a new environment? If this could be established, we could see the advantage of sexual reproduction over the non-sexual method. It would be extremely rash at present to make a generalization of this kind, for there are many forms known in which the only method of propagation that exists is the non-sexual one. In other words, there are no grounds for the assumption that this is a necessary condition for all kinds of protoplasm, but only for certain kinds.

In the insects, crustaceans, rotifers, and in some plants there are a few species whose egg develops without fertilization. This makes it appear probable that the particular kind of protoplasm of these animals does not absolutely require union from time to time with the protoplasm of another individual having a somewhat different constitution.

There is also an interesting parallel between the effects of solutions on the protozoans in Calkins’s experiments and certain results that have been obtained in artificial parthenogenesis. It has been stated, that by brushing the unfertilized eggs of the silkworm moth a larger percentage will develop parthenogenetically; and more recently it has been shown by Matthews that by agitation of the water in which the unfertilized eggs of the starfish have been placed many of them will begin their development. It was first shown by Richard Hertwig that by putting the unfertilized eggs of the sea-urchin in strychnine solutions, they will begin to segment, and I obtained the same results much better by placing the eggs in solutions of magnesium chloride. Loeb then succeeded in carrying the development to a later stage by using a different strength of the same solution, as well as by other solutions. Under the most favorable circumstances some of the eggs may produce larvÆ that seem normal in all respects, but whether they can develop into adult sea-urchins has not yet been shown.

These results indicate that one at least of the factors of fertilization is the stimulus given to the egg. On the other hand, the lack of vigor shown by many eggs that have been artificially fertilized indicates that some other result is also accomplished by the normal method of fertilization that is here absent. This may mean no more than that as yet we have not found all the conditions necessary to supply the place of the spermatozoon.

In our study of the phenomena of adaptation we have found that sometimes the adaptation is for the benefit of the individual and at other times for the benefit of the species. May it not be true also that the process of sexual reproduction has more to do with a benefit conferred on the race rather than on the individual? In fact, Weismann has elaborated a view based on the conception that the process of sexual reproduction is beneficial to the race rather than to the individual. His idea, however, is not so much that the result is of direct benefit to a particular species, as it is advantageous to the formation of new species from the original one. In a sense this amounts, perhaps, to nearly the same thing, but in another sense the idea involves a somewhat different point of view.

According to his view “the deeper significance of conjugation” and of sexual reproduction is concerned “with the mingling of the hereditary tendencies of two individuals.” In this way, through the different combinations that are formed, variations which he supposes are indispensable for the action of natural selection originate. The purpose of the sexual process is solely, according to Weismann, to supply the variations for natural selection. If it be asked how this process has been acquired for the purpose of supplying natural selection with the material on which it can work, we find the following reply given by Weismann. “But if amphimixis [by which he means the union of sex-cells from different individuals] is not absolutely necessary, the rarity of purely parthenogenetic reproduction shows that it must have a widespread and deep significance. Its benefits are not to be sought in the single individual; for organisms can arise by agamic methods, without thereby suffering any loss of vital energy; amphimixis must rather be advantageous for the maintenance and modification of species. As soon as we admit that amphimixis confers some such benefits, it is clear that the latter must be augmented, as the method appears more frequently in the course of generations; hence we are led to inquire how nature can best have undertaken to give this amphimixis the widest possible range in the organic world.” Nature, Weismann says, could find no more effectual means of bringing about the union of the sexual cells than by rendering them incapable of developing alone. “The male germ-cells, being specially adapted for seeking and entering the ovum, are, as a rule, so ill provided with nutriment that their unaided development into an individual would be impossible; but with the ovum it is otherwise, and accordingly the ‘reduction division’ removes half the germ-plasm and the power of developing is withdrawn.” It can scarcely be claimed, in the light of more recent discoveries, that the reduction division takes place in order to prevent the development of the ovum, for how then could we explain the corresponding division of the male germ-cells?

Whatever means has been employed to bring about the process of sexual reproduction, the guiding principle is supposed by Weismann to be natural selection as stated in the following paragraph: “If we regard amphimixis as an adaptation of the highest importance, the phenomenon can be explained in a simple way. I only assume that amphimixis is of advantage in the phyletic development of life, and furthermore that it is beneficial in maintaining the level of adaptation, which has been once attained, in every single organism; for this is as dependent upon the continuous activity of natural selection as the coming of new species. According to the frequency with which amphimixis recurs in the life of a species, is the efficiency with which the species is maintained; since so much the more easily will it adapt itself to new conditions of life, and thus become modified.”

Thus we reach the somewhat startling conclusion that through natural selection the germ-cells and their protozoan prototypes have been rendered incapable, through natural selection, of reproducing by non-sexual methods, in order that variations may be supplied for the farther action of this same process of natural selection. The speculation has the appearance of arguing in a circle, although if it were worth the attempt an ingenious mind might perhaps succeed in showing that such a thing is not logically inconceivable.

It seems strange that a claim of this sort should have been made, when it is so apparent that the most immediate effect of intercrossing is to swamp all variations that depart from the average. Even if it were true that new combinations of characters would arise through the union of the germ-cells of two different animals, it is certainly true that in the case of fluctuating variations this new combination would be lost by later crossing with average individuals. Moreover, it is well known that variations occur amongst forms that are produced asexually. On the whole, it does not seem to be a satisfactory solution of the problem to assume that sexual reproduction has been acquired in order to supply natural selection with material on which it may work.

Our examination of the suggestions that have been made and of the speculation indulged in, as to what benefit the process of sexual reproduction confers on the animals and plants that make use of this method of propagation, has failed to show convincingly that any advantage to the individual or to the species is the outcome. This may mean, either that there is no advantage, or that we have as yet failed to understand the meaning of the phenomenon. The only light that has been thrown on the question is that a certain amount of renewed vigor is a consequence of this process, but we cannot explain how this takes place. There is also the suggestion that the union of different cells produces the same beneficial effect as a change in the conditions of life produces on the organism. The bad effects of close interbreeding that seem sometimes to follow is explicable on this view. This, it seems to me, is the most plausible solution of the question that has been advanced; but, even if this should prove to be the correct view, we need not assume that the process has been acquired on account of this advantage, for there is nothing to show that it has been acquired in this way.