  The two terms continuous and discontinuous variation refer to the succession or inheritance of the variations rather than to the actual conditions amongst a group of individuals living at the same time; but this distinction has only a subordinate value. The term fluctuating, or individual variation, expresses more nearly the conditions of the individuals of a species at any one time, and the continuation of this sort of difference is the continuous variation spoken of above. The discontinuous variations are probably of the same nature as those that have been called mutations, and what Darwin sometimes called sports, or single variations, or definite variations. Continuous VariationIf we examine a number of individuals of the same species, we find that no two of them are exactly alike in all particulars. If, however, we arrange them according to some one character, for example, according to the height, we find that there is a gradation more or less perfect from one end of the series to the other. Thus, if we were to take at random a hundred men, and stand them in line arranged according to their height, the tops of their heads, if joined, would form a nearly continuous line; the line will, of course, incline downward from the tallest to the shortest man. This illustrates individual variation. An arrangement of this kind fails to bring out one of the most important facts connected with individual differences. Fig. 3.—Curves of frequency, etc. “In arranging the individuals it will be found, as has been said, that certain groups contain more individuals. They will form the longest line. This value that occurs with the greatest frequency is called the mode. The position of this modal class in the polygon is one of the points of importance, and the spread of the polygon at its base is another. A polygon with a low mode and a broad range means great variability. The range may, however, be much affected by a 21.Davenport, C. B., “The Statistical Study of Biological Problems,” Popular Science Monthly, September, 1900. Of the different kinds of polygons there are two main sorts, the simple and the complex. The former have only a single mode, the latter have more than one mode. Some simple polygons lie symmetrically on each side of the mode, Figure 3 A; others are unsymmetrical or skew, Figure 3 B. The skew polygon generally extends out on one side farther than on the other. It has been suggested that when a polygon is symmetrical the species is not changing, and when skew that the species is evolving in the direction of the longer base. This assumes that the sort of variation measured by these curves is of the kind of which evolution is made up, but this is a question that we must further consider. How far the change indicated by the skew curve may be carried is also another point for further examination. A complex polygon of variation, Figure 3 D, has been sometimes interpreted to mean that two subgroups exist in a species, as is well shown in the case of the rhinoceros beetle described by Bateson. Two kinds of male individuals exist, some with long horns, others with short horns; each with a mode of its own, the two polygons overlapping. Other complex polygons may be due to changes occurring at different times in the life of the individual, as old age, for example. If, instead of examining the variations of the individuals of the race, we study the variations in the different organs of the same individual, we find in many cases that certain organs vary together. Thus the right and the left leg nearly always vary in the same direction, also the first joints of the index and middle fingers, and the stature and the forearm. On the other hand, the length of the clavicle and that of the humerus do not vary together to the same extent; and the breadth and height of the skull even less so.
We may also study those cases in which a particular organ is repeated a number of times in the same individual, as are the leaves of a tree. If the leaves of the same tree are examined in respect, for example, to the number of veins that each contains, we find that the number varies, and that the results give a variation polygon exactly like that when different individuals are compared with one another. Let us take the illustration given by Pearson. He counted the veins on each side of the midrib of the leaves of the beech. If a number of leaves be collected from one tree, and the same number from another, and if all those having fifteen veins are put in one vertical column, and all those with sixteen in another, as shown in the following table, it will be found that each tree has a mode of its own. Thus in the first tree the mode is represented by nine individuals having eighteen veins, and in the second by nine individuals having fifteen veins. So far as this character is concerned we might have interchanged certain of the individual leaves, but we could not have interchanged the two series. They are individual to the two trees. Now in what does this individuality consist? Clearly there are most leaves in one tree with eighteen ribs, and most in the other with fifteen ribs. If we contrast these results with those obtained by picking at random a large number of leaves from different beech trees, we have no longer types of individuals, but racial characters. Pearson has given the following table to illustrate these points: Frequency of Different Types of Beech Leaves
There is some evidence showing that by selecting particular individuals of a series, and breeding from them, the mode may be changed in the direction of selection. Thus it has been stated by Davenport that the descendants of twelve- and thirteen-rayed daisies give a polygon with a skewness of +1.92; while the descendants of twenty-one-rayed plants give a polygon with a skewness of -.13. Pearson has described very concisely the possibilities involved in the selective action of the environment. He states that if we examine the frequency distribution of a set of organisms that have just become mature, and later make a similar examination on the same number of individuals (but not the same individuals) during the period of reproduction, we shall probably find that a change has taken place which may have been due to selection of some sort. The same thing might be found in the next generation, and, if it did, this would indicate that “selection does not necessarily mean a permanent or a progressive change.” The selection in this imaginary case would be purely periodic and suffice only to Pearson believes that “if a race has been long under the same environment it is probable that only periodic selection is at work, maintaining its stability. Change the environment and a secular change takes place, the deviations from the mode previously destroyed giving the requisite material.” “Clearly periods of rapidly changing environment, of great climatological and geological change, are likely to be associated with most marked secular selection. To show that there is little or no change year by year in the types of rabbit and wild poppy in our English fields, or of daphnia in our English ponds, is to put forward no great argument for the inefficiency of natural selection. Take the rabbit to Australia, the wild poppy to the Cape, the daphnia into the laboratory, and change their temperature, their food supply, and the chemical constituents of water and air, and then the existence of no secular selection would indeed be a valid argument against the Darwinian theory of evolution.” In regard to the last point, it should be noted that, even if under the changed conditions a change in the mode took place, as Pearson assumes, it does not follow necessarily that selection has had anything to do with it, but the environment may have directly changed the forms. Furthermore, and this is the essential point, even if selection does act to the extent of changing the mode, we should not be justified in concluding that this sort of change could go on increasing as long as the selection lasts. All that might happen would be to keep the species up to the highest point to which fluctuating variation can be held. This need not lead to the formation of new species, or direct the course of evolution. We need not discuss the various suggestions that have been made to explain this difficulty, none of which, as Pearson points out, have been satisfactory. He himself believes that a process of segregation of like individuals must occur, during the incipient stages at least, in the formation of species. Afterwards a correlation may exist between the new organs and the germ-cells, of such a sort that a relative or an absolute sterility between the incipient species is attained. After this condition has been reached the two new species may freely intermix without a return to the primitive type, since they are no longer fertile inter se. It seems to me, also, that this would be an essential requisite if we assume that species are slowly formed out of races from individual differences, as Pearson supposes to be the case. There are, however, other possibilities that Pearson does not take into account, namely, that from the very beginning the change may be so great that the new form is not fertile with the original one; and there is also another possibility as well, that, although the In support of his main thesis Pearson gives certain data in respect to preferential mating in the human race. By this is meant that selection of certain types of individuals is more likely to take place, and also that the fertility of certain types of individuals is greater than that of other types. The calculations are based on stature, color of hair, and of eyes. The results appear to show in all cases examined that there is a slight tendency to form new races as the result of the more frequent selection of certain kinds of individuals. But even if this is the case, what more do the results show than that local races may be formed,—races having a certain mode for height, for color of eyes or of hair? That changes of this kind can be brought about we knew already without any elaborate measurements, yet we should not conclude from this that new species will be formed by a continuation of the process. Pearson writes: “As to the problem of evolution itself we are learning to see it under a new light. Natural selection, combined with sexual selection [by which Pearson means segregation of certain types through individual selection] and heredity, is actually at work changing types. We have quantitative evidence of its effects in many directions.” Yes! but no evidence that selection of this sort can do anything more than keep up the type to the upper limit attained in each generation by fluctuating variations. Pearson adds, “Variations do not occur accidentally, or in isolated instances; autogamic and assortive mating are realities, and the problem of the near future is not whether Darwinism is a reality, but Heredity and Continuous VariationIt has been to a certain extent assumed in the preceding pages that both parents are alike, or, if different, that they have an equal influence on the offspring. This may be true in many cases for certain characteristics. Thus a son from a tall father and a short mother may be intermediate in height, or if the father is white and the mother black, the children are mulattoes. But other characters rarely or never blend. In such cases the offspring is more like one or the other parent, in which case the inheritance is said to be exclusive. Thus if one parent has blue eyes and the other black, some of the children may have black eyes and others blue. There are also cases of particular inheritance where there may be patches of color, some like the color of one parent, some like that of the other parent. The latter two kinds of inheritance will be more especially considered in the subsequent part of this chapter; for the present we are here chiefly concerned with blended characters. How much in such cases does each parent contribute to the offspring? This has been expressed by Galton in his law of ancestral heredity. This law takes into account not only the two parents, but also the four grandparents, and the eight great-grandparents, etc. There will be 1024 in the tenth generation. These 1024 individuals may be taken as a fair sample of the general population, provided there has not been much interbreeding. Are we then to look upon the individual as the fused or blended product of the population a few generations back? If this were true, should we not expect As a result of his studies on the stature of man, and on the coat color of the Basset hounds, Galton has shown that the inheritance from the parents can be represented by the fraction 1/2; that is one-half of the peculiarities of the individual comes from the two parents. The four grandparents together count for 1/4 of the total inheritance, the great-grandparents 1/8, and so on, giving the series 1/2, 1/4, 1/8. Pearson, taking certain other points into consideration, believes the following series more fully represents the inheritance from the ancestors, .3, .15, .075, .0375, etc. He concludes that, “if Darwinism be the true view of evolution, i.e. if we are to describe evolution by natural selection combined with heredity, then the law which gives us definitely and concisely the type of the offspring in terms of the ancestral peculiarities is at once the foundation stone of biology and the basis upon which heredity becomes an exact branch of science.” The preceding statements give some idea of what would occur in a community in which no selection was taking place. The results will be quite different, although the same general law of inheritance will hold, if selection takes place in each generation. If, for instance, selection takes place, the offspring after four generations will have .93 of the selected character, and without further selection will not regress, but breed true to this type. 22.In this statement the earlier ancestors are assumed to be identical with the general type of the population. If, however, we do not assume that the ancestors were mediocre, it is found that after six generations of selection the offspring will breed true to the selected type within one per cent of its value. Thus, if selection were to act on a race 23.Quoted from Pearson’s “Grammar of Science.” Thus we have exact data as to what will happen on the average when blended, fluctuating variations are selected. Important as such data must always be to give us accurate information as to what will occur if things are left to “chance” variations, yet if it should prove true that evolution has not been the outcome of chance, then the method is entirely useless to determine how evolution has occurred. More important than a knowledge of what, according to the theory of chances, fluctuating variations will do, will be information that would tell us what changes will take place in each individual. In this field we may hope to obtain data no less quantitative than those of chance variations, but of a different kind. A study of some of the results of discontinuous variation will show my meaning more clearly. Discontinuous VariationGalton, in his book on “Natural Inheritance,” points out that “the theory of natural selection might dispense with a restriction for which it is difficult to see either the need or the justification, namely, that the course of evolution always proceeds by steps that are severally minute and that become effective only through accumulation.” An apparent reason, it is suggested, for this common belief “is founded on the fact that whenever search is made for intermediate forms between widely divergent varieties, whether they are of plants or of animals, of weapons or utensils, of customs, religion, or language, or of any other product of evolution, a long and orderly series can usually be made out, each member of which differs in an almost imperceptible degree from the adjacent Bateson, also, in his “Materials for the Study of Variation,” speaks of the two possible ways in which variations may arise. He points out that it has been tacitly assumed that the transitions have been continuous, and that this assumption has introduced many gratuitous difficulties. Chief of these is the difficulty that in their initial and imperfect stages many variations would be useless. “Of the objections that have been brought against the Theory of Natural Selection, this is by far the most serious.” He continues: “The same objection may be expressed in a form which is more correct and comprehensive. We have seen that the differences between species on the whole are Specific, and are differences of kind forming a discontinuous Series, while the diversities of environment to which they are subject are, on the whole, differences of degree, and form a continuous Series; it is, therefore, hard to see how the environmental differences can thus be made in any sense the directing cause of Specific differences, which by the Theory of Natural Selection they should be. This objection of course includes that of the utility of minimal Variations.” “Now the strength of this objection lies wholly in the supposed continuity of the process of Variation. We see all There is a fair number of cases on record in which discontinuous variations have been seen to take place. Darwin himself has given a number of excellent examples, and Bateson, in the volume referred to above, has brought together a large and valuable collection of facts of this kind. Some of the most remarkable of these instances have been already referred to and need only be mentioned here. The black-shouldered peacock, the ancon ram, the turnspit dog, the merino sheep, tailless and hornless animals, are all cases in point. In several of these it has been discovered that the young inherit the peculiarities of their parents if the new variations are bred together; and what is more striking, if the new variation is crossed with the parent form, the young are like one or the other parent, and not intermediate in character. This latter point raises a question of fundamental importance in connection with the origin of species. Darwin states that he knows of no cases in which, when different species or even strongly marked varieties are crossed, the hybrids are like one form or the other. They show, he believes, always a blending of the peculiarities of the two parents. Darwin has, incidentally, raised here a question of the most far-reaching import. If it should prove true, as he believes, that inheritance of this kind of discontinuous variation is also discontinuous, and that we do not get the same result when distinct species are intercrossed, or even when well-marked domestic races are interbred, then he has, indeed, placed a great obstacle in the path of those who have tried to show that new species have arisen through discontinuous variation of this sort. If wild species, when crossed, give almost invariably intermediate forms, then it may appear that we are going against the only evidence that we can hope to obtain if we claim that discontinuous variation, of the kind that sports are made of, has supplied the material for evolution. If, furthermore, when distinct races of domesticated animals are crossed, we do not get discontinuous inheritance, it might, perhaps, with justness be claimed that this instance is paralleled by what takes place when wild species are crossed. And if domesticated forms have been largely the result of the selection of fluctuating variations, as Darwin believes, then a strong case is apparently made out in favor of Darwin’s view that continuous variation has given the material for the process of evolution in nature. Whether selection or some other factor Before we attempt to reach a conclusion on this point let us analyze the facts somewhat more closely. In the first place, a number of these cases of discontinuous variation are of the nature of abnormalities. The appearance of extra fingers or toes in man and other mammals is an example of this sort. This abnormality is, if inherited at all, inherited completely; that is, if present the extra digit is perfect, and never appears in an intermediate condition, even when one of the parents was without it. The most obvious interpretation of this fact is that when the material out of which the fingers are to develop is divided up, or separated into its component parts, one more part than usual is laid down. Similarly, when a flower belonging to the triradiate type gives rise to a quadriradiate form,—as sometimes occurs,—the new variation seems to depend simply on the material being subdivided once more than usual; perhaps because a little more of it is present, or because it has a somewhat different shape. My reasons for making a surmise of this sort are based on certain experimental facts in connection with the regeneration of animals. It has been shown in several cases that it is possible to produce more than the normal number of parts by simply dividing the material so that each part becomes more or less a new whole, and the total number of parts into which the material becomes subdivided is increased. It seems not improbable that phenomena of this sort have occurred in the course of evolution, although it is, of course, possible that those characters that define species do not belong to this class of variation. To take an example. There are nine neck-vertebrÆ in some birds, but in the swan the number is twenty-five. We cannot suppose that the ancestor of the swan gradually added enough materially to make up one new vertebra and then another, In cases of this sort we can easily understand that the inheritance must be either of one kind or the other, since intermediate conditions are impossible, when it comes to the question of one or not one; but if one individual had one and another six vertebrÆ, then it would be theoretically possible for the hybrid to have three. This brings us to a question that should have been spoken of before in regard to the inheritance of discontinuous variation. It sometimes occurs that a variation, which appears in other respects to be discontinuous, is inherited in a blended form. Thus the two kinds of variation may not always be so sharply separated as one might be led to believe. There may be two different kinds of discontinuous variation in respect to inheritance, or there may be variations that are only to a greater or a less extent inherited discontinuously; and it seems not improbable that both kinds occur. This diversion may not appear to have brought us any nearer to the solution of the difficulty that Darwin’s statement has emphasized, except in so far as it may show that the lines are not so sharply drawn as may have seemed to be the case. The solution of the difficulty is, I believe, as follows:— The discontinuity referred to by Darwin relates to cases in which only a single step (or mutation) has been taken, and it is a question of inheritance of one or not one. If, however, six successive steps should be taken in the same direction, then when such a form is crossed with the original form, the hybrid may inherit only three of the steps and stand exactly midway between the parent forms; or it may inherit four, or five, or three, or two steps and stand correspondingly nearer to the one The whole question of inheritance has assumed a new aspect; first on account of the work of De Vries in regard to the appearance of discontinuous variation in plants; and secondly, on account of the remarkable discoveries of Gregor Mendel as to the laws of inheritance of discontinuous variations. Mendel’s work, although done in 1865, was long neglected, and its importance has only been appreciated in the last few years. We shall take up Mendel’s work first, and then that of De Vries. Mendel’s Law |
| Seed parent | {A form round | Pollen parent | {a form angular |
| AB | {B albumen yellow | ab | {b albumen green |
When these two forms were crossed the seeds appeared round and yellow like those of the parent, AB, i.e. these two characters dominated in the hybrid.
These figures stand almost in the relation of 9 : 3 : 3 : 1.
These seeds were sown again in the following year and gave:—
From the round yellow seeds:—
From the angular yellow seeds:—
From the round green seeds:—
From the angular green seeds:—
Thus there were 9 different kinds of seeds produced. There had been separated out at this time 38 individuals like the parent seed plant, AB, and 30 like the parent pollen plant, ab. Since these had come from similar seeds of the preceding generation they may be looked upon as pure at this time. The forms Ab and aB are also constant
Without going into further detail it may be stated that the offspring of the parent hybrids, having two pairs of differentiating characters, are represented by the series:—
This series is really a combination of the two series:—
Mendel even went farther, and used two parent varieties having three differentiating characters, as follows:—
| ABC seed parent | abc pollen plant |
| { A form round | { a form angular |
| { B albumen yellow | { b albumen green |
| { C seed-coat grey brown | { c seed-coat white |
The results, as may be imagined, were quite complex, but can be expressed by combining these series:—
In regard to the two latter experiments, in which two and three characters respectively were used, it is interesting to point out that the form of the hybrid more nearly approaches “to that one of the parental plants which possesses the greatest number of dominant characters.” If, for instance, the seed plant has short stem, terminal white flowers, and simply inflated pods; the pollen plant, on the other hand, a long stem, violet-red flowers distributed along the stem, and constricted pods,—then the hybrid resembles the seed parent only in the form of the pod; in its other characters it agrees
Mendel points out that, from knowing the characters of the two parent forms (or varieties), one could not prophesy what the hybrid would be like without making the actual trial. Which of the characters of the two parent forms will be the dominant ones, and which recessive, can only be determined by experiment. Moreover, the hybrid characters are something peculiar to the hybrid itself, and to itself alone, and not simply the combination of the characters of the two forms. Thus in one case a hybrid from a tall and a short variety of pea was even taller than the taller parent variety. Bateson lays much emphasis on this point, believing it to be an important consideration in all questions relating to hybridization and inheritance.
The theoretical interpretation that Mendel has put upon his results is so extremely simple that there can be little doubt that he has hit on the real explanation. The results can be accounted for if we suppose that the hybrid produces egg-cells and pollen-cells, each of which is the bearer of only one of the alternative characters, dominant or recessive as the case may be. If this is the case, and if on an average there are the same number of egg-cells and pollen-cells, having one or the other of these kinds of characters, then on a random assortment meeting of egg-cells and pollen-cells, Mendel’s law would follow. For, 25 per cent of dominant pollen grains would meet with 25 per cent dominant egg-cells; 25 per cent recessive pollen grains would meet with
Or more simply by this scheme:—
Mendel’s results have received confirmation by a number of more recent workers, and while in some cases the results appear to be complicated by other factors, yet there can remain little doubt that Mendel has discovered one of the fundamental laws of heredity.
It has been found that there are some cases in which the sort of inheritance postulated by Mendel’s law does not seem to hold, and, in fact, Mendel himself spoke of such cases. He found that some kinds of hybrids do not break up in later generations into the parent forms. He also points out that in cases of discontinuity the variations in each character must be separately regarded. In most experiments in crossing, forms are chosen which differ from each other in a multitude of characters, some of which are continuous and others discontinuous, some capable of blending with their contraries while others are not. The observer in attempting to discover any regularity is confused by the complications thus introduced. Mendel’s law could only appear in such cases by the use of an overwhelming number of examples which are beyond the possibilities of experiment.
25.This statement is largely taken from Bateson’s book.
Let us now examine the bearing of these discoveries on
In the first place, Mendel’s results indicate that we cannot make any such sharp distinction as Darwin does between the results of inheritance of discontinuous and of continuous variations. As Mendel’s results show, it is the separate characters that must be considered in each case, and not simply the sum total of characters.
The more general objection that Darwin has made may appear to hold, nevertheless. He thinks that the evolution of animals and plants cannot rest primarily on the appearance of discontinuous variations, because they occur rarely and would be swamped by intercrossing. If Mendel’s law applies to such cases, that is, if a cross were made between such a sport and the original form, the hybrid in this case, if self-fertilized, would begin to split up into the two original forms. But, on the other hand, it could very rarely happen that the hybrid did fertilize its own eggs, and, unless this occurred, the hybrid, by crossing with the parent forms in each generation, would soon lose all its characters inherited from its “sport” ancestor. Unless, therefore, other individuals gave rise to sports at the same time, there would be little chance of producing new species in this way. We see then that discontinuity in itself, unless it involved infertility with the parent species, of which there is no evidence, cannot be made the basis for a theory of evolution, any more than can individual differences, for the swamping effect of intercrossing would in both cases soon obliterate the new form. If, however, a species begins to give rise to a large number of individuals of the same kind through a process of discontinuous variation, then it may happen that a new form may
The Mutation Theory of De Vries
De Vries defines the mutation theory as the conception that “the characters of the organism are made up of elements (‘Einheiten’) that are sharply separated from each other. These elements can be combined in groups, and in related species the same combinations of elements recur. Transitional forms like those that are so common in the external features of animals and plants do not exist between the elements themselves, any more than they do between the elements of the chemist.”
This principle leads, De Vries says, in the domain of the descent theory to the conception that species have arisen from each other, not continuously, but by steps. Each new step results from a new combination as compared with the old one, and the new forms are thereby completely and sharply separated from the species from which they have come. The new species is all at once there; it has arisen from the parent form without visible preparation and without transitional steps.
The mutation theory stands in sharp contrast to the selection theory. The latter uses as its starting-point the common
De Vries recognizes the following kinds of variation:—
First, the polymorphic forms of the systematists. The ordinary groups which, following LinnÆus, we call species, are according to De Vries collective groups, which are the outcome of mutations. Many such LinnÆan species include small series of related forms, and sometimes even large numbers of such forms. These are as distinctly and completely separated from each other as are the best species. Generally these small groups are called varieties, or subspecies,—varieties when they are separated by a single striking character, subspecies when they differ in the totality of their characters, in the so-called habitus.
These groups have already been recognized by some investigators as elementary species, and have been given corresponding binary names. Thus there are recognized two hundred elementary species of the form formerly called Draba verna.
When brought under cultivation these elementary species are constant in character and transmit their peculiarities truly. They are not local races in the sense that they are the outcome in each generation of special external conditions.
Second, the polymorphism due to intercrossing is the outcome of different combinations of hereditary qualities. There are here, De Vries says, two important classes of facts to be kept strictly apart,—scientific experiment, and the results of the gardener and of the cultivator. The experimenter chooses for crossing, species as little variable as possible; the gardener and cultivator on the other hand prefer to cross forms of which one at least is variable, because the variations may be transmitted to the hybrid, and in this way a new form be produced.
New elementary characters arise in experiments in crossing only through variability, not through crossing itself.
Third, variability in the ordinary sense, that is, individual variability, includes those differences between the individual organs that follow Quetelet’s theory of chance. This kind of variability is characterized by its presence at all times, in all groups of individuals.
De Vries recalls Galton’s apt comparison between variability and a polyhedron which can roll from one face to another. When it comes to rest on any particular face, it is in stable equilibrium. Small vibrations or disturbances may make it oscillate, but it returns always to the same face. These oscillations are like the fluctuating variations. A greater disturbance may cause the polyhedron to roll over on to a new face, where it comes to rest again, only showing the ever present fluctuations around its new centre. The new position corresponds to a mutation. It may appear from our familiarity with the great changes that we associate with the idea of discontinuous variability, that a mutation must also involve a considerable change. Such, however, De Vries says, is not the case. In fact, numerous mutations are smaller than the extremes of fluctuating variation. For example, the
The following example is given by De Vries to illustrate the general point of view in regard to varieties and species. The species Oxalis corniculata is a “collective” species that lives in New Zealand. It has been described as having seven well-characterized varieties which do not live together or have intermediate forms. If we knew only this group, there would be no question that there are seven good species. But in other countries intermediate forms exist, which exactly bridge over the differences between the seven New Zealand forms. For this reason all the forms have been united in a single species.
Another example is that of the fern, Lomaria procera, from New Zealand, Australia, South Africa, and South America. If the forms from only one country be considered, they appear to be different species; but if all the forms from the different parts of the world be taken into account, they constitute a connected group, and are united into one large species.
It will be seen, therefore, that the limits of a collective species are determined solely by the deficiencies in the genealogical tree of the elementary species. If all the elementary species in one country were destroyed, then the forms living in other countries that had been previously held together because of those which have now been destroyed, would, after the destruction, become true species. In other words: “The LinnÆan species are formed by the disappearance of other elementary species, which at first connected all forms. This mode of origin is a purely historical process, and can never become the subject of experimental investigation.” Spencer’s famous expression, the “survival of the fittest,” is incomplete, and should read the “survival of the
De Vries gives a critical analysis of a number of cases in which new races have been formed under domestication. He shows very convincingly that, whenever the result has been the outcome of the selection of fluctuating variations, the product that is formed can only be kept to its highest point of development by the most rigid and ever watchful care. If selection ceases for only a few generations, the new form sinks back at once to its original level. Many of our cultivated plants have really arisen, not by selection of this sort, but by mutations; and there are a number of recorded cases where the first and sudden appearance of a new form has been observed. In such cases as these there is no need for selection, for if left to themselves there is no return to the original form. If, however, after a new mutation has appeared in this way, we subject its fluctuating variations to selection, we can keep the new form up to its most extreme limit, but can do nothing more.
Another means, frequently employed, by which new varieties have been formed is by bringing together different elementary species under cultivation. For instance, there are a large number of wild elementary species of apples, and De Vries believes that our different races of apples owe their origin in part to these different wild forms. Crossing, cultivation, and selection have done the rest.
De Vries points out some of the inconsistencies of those who have attempted to discriminate between varieties and species. The only rule that can be adhered to is that a variety differs from a species to which it belongs in only one or in a few characters. Most so-called varieties in nature are really elementary species, which differ from their nearest
In regard to the distribution of species in nature, it may be generally stated that the larger the geographical domain so much the larger is the number of elementary species. They are found to be heaped up in the centre of their area of distribution, but are more scattered at the periphery.
In any one locality each LinnÆan species has as a rule only one or a few elementary species. The larger the area the more numerous the forms. From France alone Jordan had brought together in his garden 50 elementary species of Draba verna. From England, Italy, and Austria there could be added 150 more. This polymorphism is, De Vries thinks, a general phenomenon, although the number of forms is seldom so great as in this case.
Amongst animals this great variety of forms is not often met with, yet amongst the mammalia and birds of North America there are many cases of local forms or races, some of which at least are probably mutations. This can only be proven, however, by actually transferring the forms to new localities in order to find out if they retain their original characters, or become changed into another form. It seems not improbable that many of the forms are not the outcome of the external conditions under which the animal now lives, but would perpetuate themselves in a new environment.
From the evidence that his results have given, De Vries believes it is probable that mutation has occurred in all directions. In the same way that Darwin supposed that individual or fluctuating variations are scattering, so also De Vries believes that the new forms that arise through mutation are scattering. On this point it seems to me that De Vries may
De Vries asks whether there are for each species periods of mutation when many and great changes take place, and periods when relatively little change occurs. The evidence upon which to form an opinion is scanty, but De Vries is inclined to think that such periods do occur. It is at least certain from our experience that there are long periods when we do not see new forms arising, while at other times, although we know very few of them, epidemics of change may take place. The mutative period which De Vries found in the evening primrose is the best-known example of such a period of active mutation. Equally important for the descent theory is the idea that the same mutation may appear time after time. There is good evidence to show that this really occurs, and in consequence the chances for the perpetuation of such a form are greatly increased. Delboeuf, who advocated this idea of the repeated reappearance of a new form, has also attempted to show that if this occurs the new form may become established without selection of any kind taking place,—the
We may now proceed to examine the evidence from which De Vries has been led to the general conclusions given in the preceding pages. De Vries found at Hilversam, near Amsterdam, a locality where a number of plants of the evening primrose, Œnothera lamarckiana, grow in large numbers. This plant is an American form that has been imported into Europe. It often escapes from cultivation, as is the case at Hilversam, where for ten years it had been growing wild. Its rapid increase in numbers in the course of a few years may be one of the causes that has led to the appearance of a mutation period. The escaped plants showed fluctuating variations in nearly all of their organs. They also had produced a number of abnormal forms. Some of the plants came to maturity in one year, others in two, or in rare cases in three, years.
A year after the first finding of these plants De Vries observed two well-characterized forms, which he at once recognized as new elementary species. One of these was O. brevistylis, which occurred only as female plants. The other new species was a smooth-leafed form with a more beautiful foliage than O. lamarckiana. This is O. lÆvifola. It was found that both of these new forms bred true from self-fertilized seeds. At first only a few specimens were found, each form in a particular part of the field, which looks as though each might have come from the seeds of a single plant.
These two new forms, as well as the common O. lamarckiana, were collected, and from these plants there have arisen the three groups or families of elementary species that De Vries has studied. In his garden other new forms also arose from those that had been brought under cultivation. The largest group and the most important one is that from the original O. lamarckiana form. The accompanying table
Thus De Vries had, in his seven generations, about fifty thousand plants, and about eight hundred of these were mutations. When the flowers of the new forms were artificially fertilized with pollen from the flowers on the same plant, or of the same kind of plant, they gave rise to forms like themselves, thus showing that they are true elementary species.
26.O. lata is always female, and cannot, therefore, be self-fertilized. When crossed with O. lamarckiana there is produced fifteen to twenty per cent of pure lata individuals.
Only one form, O. scintillans, that appeared eight times, is not constant as are the other species. When self-fertilized its seeds produce always three other forms, O. scintillans, O. oblongata, and O. lamarckiana. It differs in this respect from all the other elementary species, which mutate not more than once in ten thousand individuals.
From the seeds of one of the new forms, O. lÆvifolia, collected in the field, plants were reared, some of which were O. lamarckiana and others O. lÆvifolia. They were allowed to grow together, and their descendants gave rise to the same forms found in the lamarckiana family, described above, namely, O. lata, elliptica, nannella, rubrinervis, and also two new species, O. spatulata and leptocarpa.
In the lata family, only female flowers are produced, and, therefore, in order to obtain seeds they were fertilized with
De Vries also watched the field from which the original forms were obtained, and found there many of the new species that appeared under cultivation. These were found, however, only as weak young plants that rarely flowered. Five of the new forms were seen either in the Hilversam field, or else raised from seeds that had been collected there. These facts show that the new species are not due to cultivation, and that they arise year after year from the seeds of the parent form, O. lamarckiana.
Conclusions
From the evidence given in the preceding pages it appears that the line between fluctuating variations and mutations may be sharply drawn. If we assume that mutations have furnished the material for the process of evolution, the whole problem appears in a different light from that in which it was placed by Darwin when he assumed that the fluctuating variations are the kind which give the material for evolution.
From the point of view of the mutation theory, species are no longer looked upon as having been slowly built up through the selection of individual variations, but the elementary species, at least, appear at a single advance, and fully formed. This need not necessarily mean that great changes have suddenly taken place, and in this respect the mutation theory is in accord with Darwin’s view that extreme forms that rarely appear, “sports,” have not furnished the material for the process of evolution.
As De Vries has pointed out, each mutation may be different from the parent form in only a slight degree for each point, although all the points may be different. The most unique
In consequence of this difference in the two theories, it will not be difficult to show that the mutation theory escapes some of the gravest difficulties that the Darwinian theory has encountered. Some of the advantages of the mutation theory may be briefly mentioned here.
1. Since the mutations appear fully formed from the beginning, there is no difficulty in accounting for the incipient stages in the development of an organ, and since the organ may persist, even when it has no value to the race, it may become further developed by later mutations and may come to have finally an important relation to the life of the individual.
2. The new mutations may appear in large numbers, and of the different kinds those will persist that can get a foothold. On account of the large number of times that the same mutations appear, the danger of becoming swamped through crossing with the original form will be lessened in proportion to the number of new individuals that arise.
3. If the time of reaching maturity in the new form is different from that in the parent forms, then the new species
4. The new species that appear may be in some cases already adapted to live, in a different environment from that occupied by the parent form; and if so, it will be isolated from the beginning, which will be an advantage in avoiding the bad effects of intercrossing.
5. It is well known that the differences between related species consists largely in differences of unimportant organs, and this is in harmony with the mutation theory, but one of the real difficulties of the selection theory.
6. Useless or even slightly injurious characters may appear as mutations, and if they do not seriously affect the perpetuation of the race, they may persist.
In Chapters X and XI, an attempt will be made to point out in detail the advantages which the mutation theory has over the Darwinian theory.
