One of the most important considerations in connection with the problem of adaptation is that in all animals and plants the individuals sooner or later perish and new generations take their places. Each new individual is formed, in most cases, by the union of two germ-cells derived one from each parent. As a result of this process of intermixing, carried on from generation to generation, all the individuals would tend to become alike, unless something else should come in to affect the result.

So far as our actual experience reaches, we find that the succeeding generations of individuals resemble each other. It is true that no two individuals are absolutely alike, but if a sufficiently large number are examined at a given time, they will show about the same variations in about the same proportionate numbers. Such a group of similar forms, repeating itself in each generation, is the unit of the systematists, and is called a species.

It has been said that within each species the individuals differ more or less from each other, but our experience teaches that in each generation the same kinds of variations occur, and, moreover, that from any one individual there may arise in the next generation any one of the characteristic variations. Certain limitations will have to be made in regard to this statement, but for the present it will suffice. The Law of Biogenesis states that each living thing arises from another living thing; that there is no life without antecedent life, i.e. spontaneous generation does not occur. The law is not concerned with the likeness or unlikeness of the different individuals that descend from each other. The theory of evolution includes the same idea, but in addition it has come to mean nowadays, that there have been changes, as the succeeding generations have arisen. The transmutation theory, and even the descent theory, have come to mean nearly the same thing as the theory of evolution. It is unfortunate that one of these terms cannot be used to signify simply the repetition, generation after generation, of groups of similar individuals. The theory of descent might be used to convey only this idea, but unfortunately it too has come to include also the idea of change. I shall attempt nevertheless to discriminate between the descent and the transmutation theory, and use the term descent theory when I do not wish to convey the idea of change, and transmutation theory when I do wish to emphasize this idea.

On the transmutation theory it is assumed that a group (species) may give rise to one or more groups of forms differing from their ancestors; the original group being now replaced by its new kinds of offspring, or the old and the new may remain in existence at the same time. This process repeating itself, each or some of the new groups giving rise in turn to one or more new species, there will be produced a larger group of species having certain similar characters which are due to their common descent. Such a group of species is called a genus. The resemblances of these species is accounted for by their common descent; but their differences must be due to those factors that have caused them to depart from the original type. We may now proceed to consider the evidence on which this idea of transmutation rests.

It does not require any special study to see that there are certain groups of animals and of plants that are more like each other than they are like the members of any other group. It is obvious to every one that the group known as mammals has a combination of characters not found in any other group; such, for instance, as a covering of hair, mammary glands that furnish milk to the young, and a number of other less distinctive features. These and other common characteristics lead us to put the mammals into a single class. The birds, again, have certain common characters such as feathers, a beak without teeth, the development of a shell around the egg, etc., and on account of these resemblances we put them into another class. Everywhere in the animal and plant kingdoms we find large groups of similar forms, such as the butterflies, the beetles, the annelidan worms, the corals, the snails, the starfishes, etc.

Within each of these groups we find smaller groups, in each of which there are again forms more like each other than like those of other groups. We may call these smaller groups families. Within the families we find smaller groups, that are more like each other than like any other groups in the same family, and these we put into genera. Within the genus we find smaller groups following the same rule, and these are the species. Here we seem to have reached a limit in many cases, for we do not always find within the species groups of individuals more like each other than like other groups. Although we find certain differences between the individuals of a species, yet the differences are often inconstant in the sense that amongst the descendants of any individual there may appear any one of the other variations. If this were the whole truth, it would seem that we had here reached the limits of classification, the species being the unit. This, however, is far from being the case, for, in many species we find smaller groups, often confined to special localities. These groups are called varieties.

In some cases it appears, especially in plants, these smaller groups of varieties resemble in many ways the groups of species in other forms, since they breed true to their kind, even under changed conditions. They have been recognized as “smaller species” by a number of botanists.

In this connection a point must be brought up that has played an important rÔle in all discussion as to what limits can be set to a species. As a rule it is found that two distinct species cannot be made to cross with each other, i.e. the eggs of an individual of one species cannot be fertilized by spermatozoa derived from individuals of another species; or, at least, if fertilization takes place the embryo does not develop. In some cases, however, it has been found possible to cross-fertilize two distinct species, although the offspring is itself more or less infertile. Even this distinction, however, does not hold absolutely, for, in a few cases, the offspring of the cross is fertile. It cannot be maintained, therefore, that this test of infertility between species invariably holds, although in a negative sense the test may apply, for if two different forms are infertile, inter se, the result shows that they are distinct species. If they cross they may or may not be good species, and some other test must be used to decide their relation.

We should always keep in mind the fact that the individual is the only reality with which we have to deal, and that the arrangement of these into species, genera, families, etc., is only a scheme invented by man for purposes of classification. Thus there is no such thing in nature as a species, except as a concept of a group of forms more or less alike. In nature there are no genera, families, orders, etc. These are inventions of man for purposes of classification.

Having discovered that it is possible to arrange animals and plants in groups within groups, the question arises as to the meaning of this relation. Have these facts any other significance than that of a classification of geometric figures, or of crystals according to the relations of their axes, or of bodies as to whether they are solids, liquids, or gases, or even whether they are red, white, or blue?

If we accept the transmutation view, we can offer an explanation of the grouping of living things. According to the transmutation theory, the grouping of living things is due to their common descent, and the greater or less extent to which the different forms have diverged from each other. It is the belief in this principle that makes the classification of the biologist appear to be of a different order from that in any other science; and it is this principle that appears to give us an insight into a large number of phenomena.

For example, if, as assumed in the theory, a group of individuals (species) breaks up into two groups, each of these may be supposed to inherit a large number of common characteristics from their ancestors. These characters are, of course, the resemblances, and from them we conclude that the species are related and, therefore, we put them into the same genus. The differences, as has been said, between the species must be explained in some other way; but the principle of classification with which we are here concerned is based simply on the resemblances, and takes no account of the differences between species.

In this argument it has been tacitly assumed that the transformation of one species into another, or into more than one, takes place by adding one or more new characters to those already present, or by changing over a few characters without altering others. But when we come to examine any two species whatsoever, we find that they differ, not only in one or in a few characters, but in a large number of points; perhaps in every single character. It is true that sometimes the differences are so small that it is difficult to distinguish between two forms, but even in such cases the differences, although small, may be as numerous as when they are more conspicuous. If, then, this is what we really find when we carefully examine species of animals or of plants, what is meant when we claim that our classification is based on the characters common to all of the forms that have descended from the same ancestor? We shall find, if we press this point that, in one sense, there is no absolute basis of this sort for our classification, and that we have an unreal system.

If this is admitted, does our boasted system of classification, based as it is on the principle of descent, give us anything fundamentally different from an artificial classification? A few illustrations may make clearer the discussion that follows. If, for example, we take a definition of the group of vertebrates we read: “The group of craniate vertebrates includes those animals known as Fishes, Amphibians, Reptiles, Birds, and Mammals; or in other words, Vertebrates with a skull, a highly complex brain, a heart of three or four chambers, and red blood corpuscles.” If we attempt to analyze this definition, we find it stated that the skull is a characteristic of all vertebrates, but if we ask what this thing is that is called skull, we find not only that it is something different in different groups, being cartilaginous in sharks, and composed of bones in mammals, but that it is not even identical in any two species of vertebrates. If we try to define it as a case of harder material around the brain, then it is not something peculiar to the vertebrates, since the brain of the squid is also encased in a cartilaginous skull. What has been said of the skull may be said in substance of the brain, of the heart, and even of the red blood corpuscles.

If we select another group, we find that the birds present a sharply defined class with very definite characters. The definition of the group runs as follows: “Birds are characterized by the presence of feathers, their fore-limbs are used for flight, the breast-bone is large and serves for the attachment of the muscles that move the wings; outgrowths from the lungs extend throughout the body and even into the bones and serve as air sacs which make the body more buoyant. Only one aortic arch is present, the right, and the right ovary and oviduct are not developed. The eyes are large and well developed. Teeth are absent. We have here a series of strongly marked characteristics such as distinguish hardly any other class. Moreover, the organization of existing birds is, in its essential features, singularly uniform; the entire class presenting less diversity of structure than many orders of Fishes, Amphibians, and Reptiles.”^[1] The feathers are the most unique features of birds, and are not found in any other group of the animal kingdom; moreover the plan on which they are formed is essentially the same throughout the group, yet in no two species are the feathers identical, but differ not only in form and proportions, but even in the character of the barbs and hooks for holding the vane together. The modification of the fore-limbs for flight is another characteristic feature; yet in some birds, as the ostrich and kiwi, although the wing has the same general plan as in other birds, it is not used for flight. In the latter it is so small that it does not project beyond the feathers, and in some birds, as in the penguins, the wings are used only as organs for swimming.

In spite of these differences we have no difficulty in recognizing throughout the group of birds a similarity of plan or structure, modified though it be in a thousand different ways.

Enough has been said to illustrate what is meant by the similarities of organisms on which we base our system of classification. When we conclude from the statement that all vertebrates have a skull that they owe this to a common descent, we do not mean that a particular structure has been handed down as a sort of entailed heirloom, but that the descendants have followed the same plan of structure as that of their ancestors, and have the brain enclosed in a covering of harder material, although this material may not have exactly the same form, or be made of the same substance in all cases. Furthermore while we may recognize that the cartilaginous skull of the shark is simpler in structure than that of the cartilaginous-bony skull of the frog, and that the skull of the frog is simpler than that of the rabbit, yet we should not be justified in stating, except in a metaphorical sense, that something has been added to the skull of the shark to make that of the frog, and something to the latter to make that of the rabbit. On the contrary, while something may have been added, and the plan made more complicated, the skull has also been changed throughout in every single part.

There is another point of some importance to be taken into account in this connection; namely, that each new generation begins life as a single cell or egg. The egg does not contain any preformed adult structures that it hands down unaltered, but it is so constructed that, under constant conditions, the same, or nearly the same, kind of structure is produced. Should something affect the egg, we can imagine that it might form a new combination on the same general plan as that of the old, yet one that differed from the original in every detail of its structure. It is this idea, I believe, that lies at the base of the transmutation theory. On some such assumption as this, and on this alone, can we bring the theory of transmutation into harmony with the facts of observation.

What has been said in regard to individuals as a whole may be repeated also in respect to the study of the single organs. Selecting any one group of the animal or plant kingdom, we find the same organ, or the same combination of organs present in whole groups of forms. We can often arrange these organs in definite series passing from the simple to the complex, or, in case of degeneration, in the reverse order. However convenient it may be to study the structure of organisms from this point of view, the artificiality of the procedure will be obvious, since here also the organs of any two species do not differ from each other in only one point, but in many, perhaps in all. Therefore to arrange or to compare them according to any one scheme gives only an incomplete idea of their structure. We should apply here the same point of view that we used above in forming a conception of the meaning of the zoological and botanical systems. We must admit that our scheme is only an ideal, which corresponds to nothing real in nature, but is an abstraction based on the results of our experience. It might be a pleasing fancy to imagine that this ideal scheme corresponds to the plan of structure or of organization that is in every egg, and furnishes the basis for all the variations that have come or may come into existence; but we should find no justification whatsoever for believing that our fiction corresponds to any such real thing.

To sum up the discussion: we find that the resemblances of animals and plants can be accounted for on the transmutation theory, not in the way commonly implied, but in a somewhat different sense. We have found that the resemblances between the different members of a group are only of a very general sort, and the structures are not identically the same in any two species—in fact, perhaps in no two individuals. This conclusion, however, does not stand in contradiction to the transmutation hypothesis, because, since each individual begins as an egg which is not a replica of the original adult from which it is derived, there can be no identity, but at most a very close similarity. Admitting, then, that our scheme is an ideal one, we can claim, nevertheless, that on this basis the facts of classification find a legitimate explanation in the transmutation theory.

On the theory of descent, as well as on the theory of transmutation, the ancestors of all present forms are supposed to have lived at some time in the past on the surface of the earth. If, therefore, their remains should have been preserved, we should expect on the descent theory to find some, at least, of these remains to be like present forms, while on the transmutation theory we should expect to find most, if not all, of the ancestral forms to be different from the present ones.

The evidence shows that fossil forms are practically all different from living forms, and the older they are the greater the difference from present forms. In general, therefore, it may be said that the evidence is in favor of the transmutation theory. It can scarcely be claimed that the evidence is absolutely conclusive, however probable it may appear, for the problem is complicated in a number of ways.

In the first place, there is convincing evidence that some forms have been entirely exterminated. Other groups have very few living representatives, as is the case in the group containing nautilus, and in that of the crinoids. It is therefore always possible that a given fossil form may represent an extinct line, and may be only indirectly connected with forms alive at the present time. Again the historical record is so broken and incomplete in all but a few cases that its interpretation is largely a question of probability. We can easily conceive that it would be only in very exceptional cases that successive generations of the same form would be buried one above the other, so that we should find the series unbroken. This is evident not only because the conditions that were at one time favorable for the preservation of organic remains might not be favorable at another time, but also because if the conditions remained the same the organisms themselves might also remain unchanged. A new form, in fact, would be, ex hypothese, better suited to live in a different environment, and consequently we should not expect always to find its remains in the same place as that occupied by the parent species. This possibility of migration of new forms into a new locality makes the interpretation of the geological record extremely hazardous.

Nevertheless, if the evolution of the entire animal and plant kingdoms had taken place within the period between the first deposits of stratified rocks and the present time, we might still have expected to find, despite the imperfections of the record, sufficient evidence to show how the present groups have arisen, and how they are related to one another. But, unfortunately, at the period when the history of the rocks begins, nearly all the large groups of animals were in existence, and some of them, indeed, as the trilobites and the brachiopods, appear to have reached the zenith of their development.

On the other hand, the subdivisions of the group of vertebrates have evolved during the period known to us. It is true that the group was already formed when our knowledge of it begins, but, from the fishes onwards, the history of the vertebrates is recorded in the rocks. The highest group of all, the mammals, has arisen within relatively modern times. The correctness of the transmutation theory could be as well established by a single group of geological remains as by the entire animal kingdom. Let us, therefore, examine how far the theory is substantiated by the paleontological record of the vertebrates. We find that the earliest vertebrates were fishes, and these were followed successively by the amphibians, reptiles, birds, and mammals, one of the last species of all to appear being man himself. There can be little doubt that this series, with certain limitations to be spoken of in a moment, represents a progressive series beginning with the simpler forms and ending with the more complicated. Even did we not know this geological sequence we would conclude, from the anatomical evidence alone, that the progression had been in some such order as the geological record shows. The limitation referred to above is this: that while the mammals arose later than the birds, we need not suppose that the mammals arose from the birds, and not even perhaps from the reptiles, or at least not from reptiles like those living at the present day. The mammals may in fact, as some anatomists believe, have come direct from amphibian-like forms. If this is the case, we find the amphibians giving rise on one hand to reptiles and these to birds, and on the other hand to mammals.

This case illustrates how careful we should be in interpreting the record, since two or more separate branches or orders may arise independently from the same lower group. If the mammals arose from the amphibians later than did the reptiles, it would be easy to make the mistake, if the record was incomplete at this stage, of supposing that the mammals had come directly from the reptiles.

That the birds arose as an offshoot from reptile-like forms is not only probable on anatomical grounds, but the geological record has furnished us with forms like archÆopteryx, which in many ways appears to stand midway between the reptiles and birds. This fossil, archÆopteryx, has a bird-like form with feathered wings, and at the same time has a beak with reptilian teeth, and a long, feathered tail with a core of vertebrÆ.

From another point of view we see how difficult may be the interpretation of the geological record, when we recall that throughout the entire period of evolution of the vertebrates the fishes, amphibians, reptiles, and birds remained still in existence, although they, or some of them, may have at one time given origin to new forms. In fact, all these groups are alive and in a flourishing condition at the present time. The fact illustrates another point of importance, namely, that we must not infer that because a group gives rise to a higher one, that it itself goes out of existence, being exterminated by the new form. There may be in fact no relation whatsoever between the birth of a new group and the extermination of an old one.

On the transmutation theory we should expect to find not only a sequence of forms, beginning with the simplest and culminating with the more complex, but also, in the beginning of each new group, forms more or less intermediate in structure. It is claimed by all paleontologists that such forms are really found. For example, transitional forms between the fishes and the amphibia are found in the group of dipnoans, or lung-fishes, a few of which have survived to the present day. There are many fossil forms that have characters between those of amphibians and reptiles, which if not the immediate ancestors of the reptiles, yet show that at the time when this group is supposed to have arisen intermediate forms were in existence. The famous archÆopteryx remains have been already referred to above, and it appears in this case that we have not only an intermediate form, but possibly a transitional one. In the group of mammals we find that the first forms to appear were the marsupials, which are undoubtedly primitive members of the group.

The most convincing evidence of transmutation is found in certain series of forms that appear quite complete. The evolution of the horse series is the most often cited. As this case will be discussed a little later, we need not go into it fully here. It will suffice to point out that a continuous series of forms has been found, that connect the living horses having a single toe through three-toed, with the five-toed horses. Moreover, and this is important, this series shows a transformation not only in one set of structures, but in all other structures. The fossil horses with three toes are found in the higher geological layers, and those with more toes in the deeper layers progressively. In some cases, at least, the fossils have been found in the same part of the world, so that there is less risk of arranging them arbitrarily in a series to fit in with the theory.

EVIDENCE FROM DIRECT OBSERVATION AND EXPERIMENT

Within the period of human history we do not know of a single instance of the transformation of one species into another one, if we apply the most rigid and extreme tests used to distinguish wild species from each other.^[2] It may be claimed that the theory of descent is lacking, therefore, in the most essential feature that it needs to place the theory on a scientific basis. This must be admitted. On the other hand, the absence of direct observation is not fatal to the hypothesis, for several reasons. In the first place, it is only within the last few hundred years that an accurate record of wild animals and plants has been kept, so that we do not know except for this period whether any new species have appeared. Again, the chance of observing the change might not be very great, especially if the change were sudden. We would simply find a new species, and could not state where it had come from. If, on the other hand, the change were very slow, it might extend over so many years that the period would be beyond the life of an individual man. In only a few cases has it been possible to compare ancient pictures of animals and plants with their prototypes living at the present time, and it has turned out in all cases that they are the same. But these have been almost entirely domesticated forms, where, even if a change had been found, it might have been ascribed to other factors. In other cases, as in the mummified remains of a few Egyptian wild animals (which have also been found to be exactly like the same animals living at the present day), it was pointed out by Geoffroy Saint-Hilaire that, since the conditions of the Egyptian climate are the same to-day as they were two thousand years ago, there is no reason to expect any change would have taken place. But waiving this assumption, we should not forget that the theory of evolution does not postulate that a change must take place in the course of time, but only that it may take place sometimes.

The position that we have here taken in regard to the lack of evidence as to the transformation of species is, perhaps, extreme, for, as will be shown in some detail in later chapters, there is abundant evidence proving that species have been seen to change greatly when the conditions surrounding them have been changed; but never, as has been stated, so far, or rather in such a way, that an actual new species that is infertile with the original form has been produced. Whether, after all, these changes due to a change in the environment are of the kind that makes new species, is also a question to be discussed later.

The experimental evidence, in favor of the transformation of species, relates almost entirely to domesticated forms, and in this case the conscious agency of man seems, in some cases, to have played an important part; but here, even with the aid of the factor of isolation, it cannot be claimed that a single new species has been produced, although great changes in form have been effected. It is clear, therefore, that we must, at present, rely on other data, less satisfactory in all respects, to establish the probability of the theory of transformation.

MODERN CRITICISM OF THE THEORY OF EVOLUTION

Throughout the whole of the nineteenth century a steady fire of criticism was directed against the theory of evolution; the names of Cuvier and of Louis Agassiz stand out preËminent in this connection, yet the theory has claimed an ever increasing number of adherents, until at the present time it is rare to find a biologist who does not accept in one form or another the general principle involved in the theory. The storm of criticism aroused by the publication of Darwin’s “Origin of Species,” was directed more against the doctrine of evolution than against Darwin’s argument for natural selection. The ground has been gone over so often that there would be little interest in going over it again. It will be more profitable to turn our attention to the latest attack on the theory from the ranks of the zoologists themselves.

Fleischmann, in his recent book, “Die Descendenztheorie,” has made a new assault on the theory of evolution from the three standpoints of paleontology, comparative anatomy, and embryology. His general method is to try to show that the recognized leaders in these different branches of biology have been led to express essentially different views on the same questions, or rather have compromised the doctrine by the examples they have given to illustrate it. Fleischmann is fond of bringing together the antiquated and generally exaggerated views of writers like Haeckel, and contrasting them with more recent views on the same subject, without making sufficient allowances for the advances in knowledge that have taken place. He selects from each field a few specific examples, by means of which he illustrates the weakness, and even, as he believes, the falsity of the deductions drawn for the particular case. For example, the plan of structure of the vertebrates is dealt with in the following way: In this group the limbs, consisting typically of a pair of fore-legs and a pair of hind-legs, appear under the form of cylindrical outgrowths of the body. In the salamander, in the turtle, in the dog, the cylindrical legs, supporting the body and serving to support it above the ground, are used also for progression. The general purpose to which the limbs are put as organs of locomotion has not interfered with an astonishing number of varieties of structure, adapted to different conditions of existence, such as the short legs used for creeping in salamanders, lizards, turtles, crocodiles; the long and thin legs of good runners, as the hoofed animals; the mobile legs of the apes used for climbing; and the parachute legs of some squirrels used for soaring. Even more striking is the great variety of hands and feet, as seen in the flat, hairy foot of the bear; the fore-foot of the armadillos, carrying long, sickle-shaped claws; the digging foot of the mole; the plump foot of the elephant, ending in a broad, flat pad with nails around the border, and without division into fingers; the hand of man and of the apes ending with fine and delicate fingers for grasping. To have discovered a general plan of structure running through such a great variety of forms was proclaimed a triumph of anatomical study.^[3]

A study of the bony structure of the limb shows that typically it consists of a single proximal bone (the humerus in the upper arm, the femur in the thigh), followed by two bones running parallel to each other (the radius and ulna in the arm and the tibia and fibula in the shank); these are succeeded in the arm by the two series of carpal bones, and in the leg by the two series of tarsal bones, and these are followed in each by five longer bones (the metacarpals and metatarsals), and these again by the series of long bones that lie in the fingers and toes. Despite the manifold variety of forms, Fleischmann admits that both the hind- and the fore-limbs are constructed on the same plan throughout the vertebrates. Even forms like the camel, in which there are fewer terminal bones, may be brought into the same category by supposing a reduction of the bones to have taken place, so that three of the digits have been lost. In the leg of the pig and of the reindeer, even a greater reduction may be supposed to have taken place. Fleischmann points out that these facts were supposed to be in full harmony with the theory of descent.

The analysis of the origin of the foot of the horse gave even better evidence, it was claimed, in favor of the theory. The foot consists of a single series of bones corresponding to the middle finger and toe. When, as sometimes happens, individual horses are found in which in addition to the single middle finger two smaller lateral fingers with small hoofs appear, the followers of the descent theory rejoiced to be able to bring this forward as a confirmation of their doctrine. The occurrence was explained as a sporadic return to an ancestral form. The naÏve exposition of the laws of inheritance that were supposed to control such phenomena was accepted without question. And when finally a large number of fossil remains were found by paleontologists,—remains showing a gradual increase in the middle finger, and a decrease in size of the lateral fingers,—it was supposed that the proof was complete; and anatomists even went so far as to hold that the original ancestor of the horse was a five-fingered animal.

This same law of type of structure was found to extend to the entire vertebrate series, and the only plausible explanation appeared to be that adopted by Darwin and his followers, namely, that the resemblance is the result of the blood-relationship of the different forms. But a simple comparison of the skeleton of the limbs if carried out without theoretical prejudice would show, Fleischmann thinks, that there is only a common style, or plan of structure, for the vertebrates. This anatomical result has about the same value as the knowledge of the different styles of historical architecture—that, for instance, all large churches of the Gothic period have certain general principles in common. The believers in the theory of descent have, however, he thinks, gone beyond the facts, and have concluded that the common plan in animals is the consequence of a common descent. “I cannot see the necessity for such a conclusion, and I certainly should unhesitatingly deny that the common plan of the Gothic churches depended on a common architect. The illustration is, however, not perfect, because the influence of the mediÆval school of stone-cutters on its wandering apprentices is well known.”

Fleischmann adds that if the descent theory is true we should expect to find that if a common plan of structure is present in one set of organs, as the limbs, it should be present in all other organs as well, but he does not add that this is generally the case.

The weakness of Fleischmann’s argument is so apparent that we need not attempt an elaborate refutation. When he says there is no absolute proof that the common plan of structure must be the result of blood-relationship, he is not bringing a fatal argument against the theory of descent, for no one but an enthusiast sees anything more in the explanation than a very probable theory that appears to account for the facts. To demand an absolute proof for the theory is to ask for more than any reasonable advocate of the descent theory claims for it. As I have tried to show in the preceding pages, the evidence in favor of the theory of descent is not absolutely demonstrative, but the theory is the most satisfactory one that has as yet been advanced to account for the facts. Fleischmann’s reference to the common plan of structure of the Gothic churches is not very fortunate for his purpose, since he admits himself that this may be the result of a common tradition handed down from man to man, a sort of continuity that is not very dissimilar in principle from that implied in the descent theory; in the latter the continuity of substance taking the place of the tradition in the other. Had the plan for each, or even for many of the churches, originated independently in the mind of each architect, then the similarity in style would have to be accounted for by a different sort of principle from that involved in the theory of descent; but as a matter of fact the historical evidence makes it probable that similar types of architecture are largely the result of imitation and tradition. Certain variations may have been added by each architect, but it is just the similarity of type or plan that is generally supposed to be the outcome of a common tradition.

Fleischmann’s attempt in the following chapter to belittle Gegenbaur’s theory of the origin of the five-fingered type of hand from a fin, like that of a fish, need not detain us, since this theory is obviously only a special application which like any other may be wrong, without in the least injuring the general principle of descent. That all phylogenetic questions are hazardous and difficult is only too obvious to any one familiar with the literature of the last thirty years.

Fleischmann devotes a long chapter to the geological evidences in connection with the evolution of the horse, and attempts to throw ridicule on the conclusions of the paleontologists by emphasizing the differences of opinion that have been advanced in regard to the descent of this form. After pointing out that the horse, and its few living relatives, the ass and the zebra, are unique in the mammalian series in possessing a single digit, he shows that by the discovery of the fossil horses the group has been simply enlarged, and now includes horses with one, three, and five toes. The discovery of the fossil forms was interpreted by the advocates of the descent theory as a demonstration of the theory. The series was arranged by paleontologists so that the five-toed form came first, then those with three and one toe, the last represented by the living horses. But the matter was not so simple, Fleischmann points out, as it appeared to be to the earlier writers, for example to Haeckel, Huxley, Leidy, Cope, Marsh. Different authors came to express different opinions in regard to the genealogical connection between the fossil forms. Several writers have tried to show that the present genus, Equus, has not had a single line of descent, but have supposed that the European horses and the original American horses had different lines of ancestry, which may have united only far back in the genus Epihippus. Fleischmann points out that the arrangement of the series is open to the criticism that it is arbitrary, and that we could equally well make up an analogous series beginning with the five-fingered hand of man, then that of the dog with the thumb incompletely developed, then the four-fingered hind-foot of the pig without a big toe and with a weak second and fifth digit, then the foot of the camel with only two toes, and lastly the foot of the horse with only one toe. It sounds strange that Fleischmann should make such a trivial reply as this, and deliberately ignore the all-important evidence with which he is, of course, as is every zoologist, perfectly conversant. Not only are there a hundred other points of agreement in the horse series, but also the geological sequence of the strata, in which some at least of the series have been found, shows that the arrangement is not arbitrary, as he implies.

Fleischmann then proceeds to point out that when the evidence from other parts of the anatomy is taken into account, it becomes evident that all the known fossil remains of horses cannot be arranged in a single line, but that there are at least three families or groups recognizable. Many of these forms are known only from fragments of their skeletons—a few teeth, for instance, in the case of Merohippus, which on this evidence alone has been placed at the uniting point of two series. At present about eight different species of living horses are recognized by zoologists, and paleontological evidence shows only that many other species have been in existence, and that even three- and one-toed forms lived together at the same time.

Fleischmann also enters a protest against the ordinary arrangement of the fossil genera Eo-, Oro-, Meso-, Merohippus in a series, for these names stand not for single species, but for groups containing no less than six species under Protohippus, fourteen under Equus, twelve under Mesohippus, and twenty under Hipparion. Fleischmann concludes: “The descent of the horses has not been made out with the precision of an accurate proof, and it will require a great deal of work before we get an exact and thorough knowledge of the fossil forms. What a striking contrast is found on examination between the actual facts and the crude hopes of the apostles of the descent theory!...”

In so far as this criticism of Fleischmann’s applies to the difficulties of determining the past history of the horse, it may be granted that he has scored a point against those who have pretended that the evidence is simple and conclusive; but we should not fail to remember that this difficulty has been felt by paleontologists themselves, who have been the first to call attention to the complexity of the problem, and to the difficulties of finding out the actual ancestors of the living representative of the series. And while we may admit that the early enthusiasts exaggerated, unintentionally, the importance of the few forms known to them, and went too far in supposing that they had found the actual series of ancestors of living horses, yet we need not let this blind us to the importance of the facts themselves. Despite the fact that it may be difficult and, perhaps, in most cases, impossible, to arrange the fossil forms in their relations to one another and to living forms, yet on an unprejudiced view it will be clear, I think, that so far as the evidence goes it is in full harmony with the theory of descent. This is especially evident if we turn our attention to a part of the subject that is almost entirely ignored by Fleischmann, and yet is of fundamental importance in judging of the result. The series of forms beginning with the five-toed horses and ending with those having a single toe has not been brought together haphazard, as Fleischmann’s comparison might lead one to suppose, but the five-fingered forms are those from the older rocks, and the three-toed forms from more recent layers. The value of this kind of evidence might have been open to greater doubt had the series been made up of forms found scattered over the whole world, for it is well known how difficult it is to compare in point of time the rocks of different continents. But in certain parts of the world, especially in North America, series of fossil horses have been found in sedimentary deposits that appear to be perfectly continuous. This series, by itself, and without regard to the point as to whether in other parts of the world other series may exist, shows exactly those results which the theory of descent postulates, and we find here, in all probability, a direct line of descent. While it may be freely admitted that no such series can demonstrate the theory of descent with absolute certainty, yet it would be folly to disregard evidence as clear as this.

In regard to the other point raised by Fleischmann concerning the large number of species of fossil horses that have existed in past times, it is obvious that while this greatly increases the difficulty of the paleontologist it is not an objection to the descent theory. In fact, our experience with living species would lead us to expect that many types have been represented at each geological period by a number of related species that may have inhabited the same country. On the descent theory, one species only in each geological period could have been in the line of descent of the present species of horse. The difficulty of determining which species (if there were several living in a given epoch) is the ancestor of the horse is increased, but this is not in itself an objection to the theory.

The descent of birds from flying reptiles is used by Fleischmann as another point of attack on the transmutation theory. The theory postulates that the birds have come from ancestors whose fore-legs have been changed into highly specialized wings. The long vertebrated tail of the ancestral form is supposed to have become very short, and long feathers to have grown out from its stump which act as a rudder during flight. Flying reptiles with winged fore-legs and a long vertebrated tail have been actually found as fossil remains, as seen in the pterodactyls and in the famous archÆopteryx. The latter, which is generally regarded either as the immediate ancestor of living birds, or at least as a closely similar form, possessed a fore-leg having three fingers ending in claws, and feathers on the forearm similar to those of modern birds. It had a long tail, like that of a lizard, but with well-developed feathers along its sides. It had pointed teeth in the horn-covered jaws. Fleischmann proceeds to point out that the resemblance of the hand of archÆopteryx to that of the reptiles is not very close, for two fingers are absent as in modern birds. The typical form of the foot is that of the bird, and is not the simple reptilian type of structure. Feathers and not scales cover the body, and give no clew as to how the feathers of birds have arisen. He concludes, therefore, that archÆopteryx, having many true bird-like characters, such as feathers, union of bones in the foot, etc., has other characters not possessed by living birds, namely, a long, vertebrated tail, a flat breastbone, biconcave vertebrÆ, etc. Therefore, it cannot be regarded as an intermediate form. Fleischmann does not point out that it is just these characters that would be postulated on the descent theory for the ancestor of the birds, if the latter arose from reptiles. Even if it should turn out that archÆopteryx is not the immediate forefather of living birds, yet the discovery that a form really existed intermediate in many characters between the reptiles and the birds is a gain for the transmutation theory. It is from a group having such characters that the theory postulates that the birds have been evolved, and to have discovered a member of such a group speaks directly and unmistakably in favor of the probability of the transmutation theory.

Fleischmann again fails to point out that the geological period in which the remains of archÆopteryx were found, is the one just before that in which the modern group of birds appeared, and, therefore, exactly the one in which the theory demands the presence of intermediate forms. This fact adds important evidence to the view that looks upon archÆopteryx as a form belonging to a group from which living birds have arisen. That a number of recent paleontologists believe archÆopteryx to belong to the group of birds, rather than to the reptiles, or to an intermediate group, does not in the least lessen its importance, as Fleischmann pretends it does, as a form possessing a number of reptilian characters, such as the transmutation theory postulates for the early ancestors of the birds.

The origin of the mammalian phylum serves as the text for another attack on the transmutation theory. Fleischmann points out that the discovery of the monotremes, including the forms ornithorhynchus and echidna, was hailed at first as a demonstration of the supposed descent of the mammals from a reptilian ancestor. The special points of resemblance between ornithorhynchus and reptiles and birds are the complete fusion of the skull bones, the great development of the vertebrÆ of the neck region, certain similarities in the shoulder girdle, the paired oviducts opening independently into the last part of the digestive tract (cloaca), and the presence of a parchment-like shell around the large, yolk-bearing egg. These are all points of resemblance to reptiles and birds, and were interpreted as intermediate stages between the latter groups and the group of mammals. In addition to these intermediate characters, ornithorhynchus possesses some distinctive, mammalian features—mammary glands and hair, for instance. Fleischmann takes the ground, in this case, that there are so many points of difference between the monotremes and the higher mammals, that it is impossible to see how from forms like these the higher groups could have arisen, and that ornithorhynchus cannot be placed as an intermediate form, a link between saurians and mammals, as the followers of the transmutation theory maintain. He shows, giving citations, that anatomists themselves are by no means in accord as to the exact position of ornithorhynchus in relation to the higher forms.

In reply to this criticism, the same answer made above for archÆopteryx may be repeated here, namely, that because certain optimists have declared the monotremes to be connecting forms, it does not follow that the descent theory is untrue, and not even that these forms do not give support to the theory, if in a less direct way. I doubt if any living zoologist regards either ornithorhynchus or echidna as the ancestral form from which the mammals have arisen. But on the other hand it may be well not to forget that these two forms possess many characters intermediate between those of mammals and reptiles, and it is from a group having such intermediate characters that we should expect the mammals to have arisen. These forms show, if they show nothing else, that it is possible for a species to combine some of the characters of the reptiles with those of the mammals; and the transmutation theory does no more than postulate the existence at one time of such a group, the different species of which may have differed in a number of points from the two existing genera of monotremes.

The origin of lung-bearing vertebrates from fishlike ancestors, in which the swim-bladder has been changed into lungs, has been pointed to by the advocates of the transmutation theory as receiving confirmation in the existence of animals like those in the group of dipnoan fishes. In these animals both gills and a swim-bladder, that can be used as a lung, are present; and through some such intermediate forms it is generally supposed that the lung-bearing animals have arisen. Fleischmann argues, however, that, on account of certain trivial differences in the position of the duct of the swim-bladder in living species, the supposed comparison is not to the point; but the issue thus raised is too unimportant to merit further discussion. Leaving aside also some even more doubtful criticisms which are made by Fleischmann, and which might be added to indefinitely without doing more than showing the credulity of some of the more ardent followers of the transmutation theory, or else the uncertainty of some of the special applications of the theory, let us pass to Fleischmann’s criticism of the problem of development.^[4]

4.The long argument of Fleischmann in regard to the origin of the fresh-water snails, as illustrated by the planorbis series, and also the origin of the nautiloid group, has been recently dealt with fully by Plate, and, therefore, need not be considered here.