Popular Science Monthly/Volume 67/May 1905/The Origin of Species Through Selection Contrasted with their Origin Through the Appearance of Definite Variations

From Wikisource
Jump to navigation Jump to search
Popular Science Monthly Volume 67 May 1905  (1905) 
The Origin of Species Through Selection Contrasted with their Origin Through the Appearance of Definite Variations by Thomas Hunt Morgan


By Professor T. H. MORGAN,


IT is a point of some interest that at the present time those zoologists and botanists, who seem willing to transfer their allegiance from Darwin's theory of natural selection to the theory of the survival of mutations, often insist that the two points of view differ, after all, only in degree and that selection is still the key note to the situation. It seems to me, on the contrary, that there is a fundamental difference between these two views, and in the hope of making this clearer I have attempted in the following pages to contrast in certain respects the applications that have been made of Darwin's theory with the implications of the newer theory of the survival of definite variations.

Attention has seldom, I believe, been called to the fact that only those theories that have been advanced to account for the evolution of animals and plants, have received wide recognition, that pretend to explain how the adaptation of the organism to its surroundings has been acquired. No such requirement is made in the case of theories of evolution of the inorganic world. On what does this difference depend? Why do we make certain demands in the case of organic evolution that we do not make for the evolution of inorganic nature? The answer in part is, that a living thing is unstable, it is easily destroyed, and it must, if it is to maintain its integrity, be able to respond to changes in the outer world in such a way as to keep the balance that makes its existence possible.

It is true that certain chemical substances are also highly unstable, but we find in them no adjustments, no regulations, for maintaining themselves, such as animals and plants exhibit. No theory, as I have said, that pretends to account for the evolution of new organisms, has been regarded as satisfactory unless it explained how the new forms acquire those adjustments that make their life possible. How perfect these adjustments must be is a question that has never been sufficiently considered, partly because of the difficulties surrounding such an examination, and partly because of the widespread belief that living things are as perfectly adapted to their environment as we can imagine possible by the adjustment of their individual parts to the surroundings in which they live. As an illustration of how such adjustments are supposed to occur I shall take a single example, from Weismann's writings, although it would be an easy matter to give endless examples, similar to this one, from the writings of other Darwinians, who, as a rule, have been only too prone to make use of the same argument in accounting for the origin of the adaptations of animals and plants.

Length of Life as an Adaptation.

Weismann has written a most interesting essay on 'The Duration of Life,' in which he attempts to show that the length of life of the individuals of a species has been regulated by natural selection of individual variations. While it may be granted that in many respects Weismann has out-Darwined Darwin, yet his method in this instance is the same as that which the Darwinians have always employed whenever an occasion occurred.

It has often been pointed out that the life of larger animals is longer than that of smaller ones, and this seems not unreasonable, since in many cases it takes a longer time for a larger animal to reach maturity; yet this relation is by no means universal, as Weismann points out, for, while it is true that an elephant may live two hundred years and a horse not more than forty, yet a man lives longer than a horse, and a cat and a toad may also live forty years. A pig may live no longer than a crayfish. Flourens tried to show that the length of life of an animal is about five times its growing period, but this does not generally hold, since a horse may live ten times as long as it takes to reach maturity.

Complexity of structure can not explain the result, because some very simple forms live to a great age. Weismann also concludes that the length of life is not determined by the 'constitution' of the animal, for, while a queen bee may live for several years, the male lives for only a few weeks. Therefore, Weismann concludes, it is 'proved that physiological considerations alone can not determine the duration of life.'

Thus by an apparent process of exclusion Weismann reaches the conclusion, 'that duration of life is really dependent upon adaptation to external conditions, that its length, whether longer or shorter, is governed by the needs of the species.' In support of this view he points out that 'life does not greatly outlast the period of reproduction except in those species that tend, their young, and as a matter of fact we find that this is the case.' How then has this regulation been brought about? Weismann's answer is that the duration depends first on the time required to reach maturity, and since the longer this time the more the chance for destruction, the number of descendents produced must be greater in proportion as the duration of the reproductive period becomes longer. This statement is obviously of no value, for a sea urchin that may grow up in a year, produces millions of eggs, while an elephant that takes thirty years to mature produces only about six young in the whole course of its life. Equally valueless, it appears to me, is Weismann's statement 'that nature does not tend to secure the longest possible life to the adult individual, but, on the contrary, tends to shorten the period of reproduction as far as possible, and with this the duration of life.'

Coming now to the main question as to how natural selection may be supposed to increase the length of life, Weismann states, that "Duration of life like every other characteristic of the organism is subject to individual fluctuation. From our experience with the human species we know that long life is hereditary. As soon as the long lived individuals of a species obtain some advantage in the struggle for existence they will probably become dominant, and those with the shortest lives will be exterminated."

Without attempting to offer an elaborate refutation of Weismann's view, I should like briefly to present the following considerations:

1. It is, of course, almost self-evident that the existence of a species is closely bound up with its powers of reproduction, but it does not follow from this that the length of life has been adjusted to come to an end when the animal can no longer reproduce itself, because it can not reproduce itself any longer.

2. On the contrary it is more probable that the same causes that have led to the cessation of the powers of reproduction may be closely associated with those that bring about a decline in the general vitality; so that while death may follow at a variable period after the power of reproduction is lost, the two processes have not been adjusted to each other by some external need, but are part of the same physiological decline.

3. In the higher animals especially, there may be thousands of immature eggs when the animal ceases to reproduce, as in the case of the human species. It would seem to be greatly to the advantage of a species to have the individual that has surmounted the dangers of youth bring all of its eggs to maturity before it dies; yet such is not the case. The eggs fail to mature not because it is to the advantage or disadvantage of the species to perish after it has set free a part of its eggs, but because the general decline of the organism brings to an end the power to ripen its eggs.

4. The natural duration of life of each species determines when its reproductive powers begin to decline, and the relation is, therefore, exactly the reverse of that which Weismann assumes; for the cessation of the reproductive power is determined by the decline of vitality and this decline is not regulated by the cessation of the power to reproduce.

5. That the process of selection of individual fluctuations can, at best, only keep the species up to an artificial standard that will be lost as soon as a rigorous process of selection ceases, has been shown by de Vries and others. The permanent inheritance of each species can not have been acquired in this way. Unless, for example, those individuals whose life is somewhat longer or shorter were being constantly destroyed in every generation the little would soon be lost that had been so laboriously gained. It is needless to point out that no such process is taking place on a scale sufficient to regulate the evolutionary process.

6. The length of life of a species is something that is as characteristic of the species as any of its structural or physiological properties. To state that the duration of life can not be supposed to be the result of physiological processes is not simply paradoxical but absurd. The paradox and also the absurdity disappear as soon as we recognize the fact that the length of life is a characteristic of each new species and is a purely physiological process. Those new elementary species whose physiological processes fulfill the conditions of a certain environment survive, those that do not perish; and there is no subsequent lengthening of one character and shortening of another, as on a sliding scale, to fit the new form in all details to its new environment. The length of life is predetermined with the advent of the new form, and is not subsequently regulated for the benefit of that particular species. From this point of view we get a simple and clear analysis of the problem, while that which Weismann maintains leads only to an unmeaning 'paradox.'

It seems to me that the method of the Darwinian school of looking upon each particular function, or structure, of the individual as capable of indefinite control through selection is fundamentally wrong. For instance, in regard to the number of eggs characteristic of each species, it is assumed that the output is also regulated by means of selection. On the contrary it appears to me that the power to produce a certain number of eggs is one of the fixed characteristics of each species that appears, and is not increased or diminished by external needs. The number of individuals that reach maturity will stand, therefore, as a measure of how far a new species is from the beginning adapted to the old environment, or to the new one in which it establishes itself. There may be a wide range of perfection in this respect, for there are some species that produce few eggs, but succeed in bringing a large number of them to maturity, and there are other species which, despite the countless number of eggs that they produce, only succeed in barely holding on to existence. Since the great majority of lower animals and plants produce large numbers of eggs we may infer that the arrangement for propagation, while it suffices to keep the species in existence, is extremely wasteful, and far from being as perfect as we can easily imagine it might become if the process could be regulated by individual selection.

An Adaptation may be More Perfect than Survival requires.

Are organisms ever more perfectly adapted in certain characters to their environment than the demands of survival require? A positive answer to this question might release us in part from the modern test of utilitarianism.

It is a well-known fact that through use many, perhaps all, parts of the body are capable of doing more than they are called upon to do during the ordinary life of the individual. The muscles through practise not only become larger and stronger, but can even be educated to do more rapid work, as seen in the fingers of the skilled pianist. The sensation of touch can be made more perfect through practise. The skin thickens wherever continued pressure is brought to bear on it. The bones will change their form, and even make new sockets under suitable conditions. The walls of the blood vessels become thicker if more blood is thrown into the blood channels. These are typical examples of what the body is capable of doing, and the responses in each case are obviously to the advantage of the individual. What is the meaning of this power to do more than the ordinary requirements of life demand?

It has been suggested that the survival of the individual is sometimes determined by its capacity to rise to extreme occasions. For example, the deer that is capable of putting on a little more speed than its fellows, is the one that escapes. But this assumption fails to meet fully the case, for, in the first place, it assumes as already present a certain amount of the very quality to be explained. In the second place a similar capacity is also present in other organs, in which it is highly improbable that the power to improve somewhat could be of sufficient importance to be decisive in a life and death struggle. It could be of little advantage for instance to have the power of improving the musical sense beyond a very low average, and no one will suppose that this has been decisive in the evolution of the race.

In other directions also we find an apparently superfluous perfection of development. It is improbable that the extraordinary adjustments of which the eye is capable have all been acquired little by little through a life and death struggle. The eye is, however, such an important organ for the welfare of the individual, that it is hard to demonstrate positively that each stage was not of great use, but for the ear it seems improbable that its perfection in certain respects could have been of vital importance for the maintenance of the race.

The symmetry of animals and of plants is also in many cases more perfect than the requirements demand. The almost exact resemblance between the right and left sides of the body, while advantageous up to a certain point, is often far more perfect than competition requires. In fact we find the Darwinians often dodging this consideration, and referring the results to 'the laws of growth' etc. But if these 'laws of growth' exist why may they not have also carried the perfection of any organ far beyond the point at which the test of survival stops?

We meet with a somewhat similar case in the distribution of color over the bodies of animals. Granting that in some cases the presence of colors, after they have appeared, may be of some use to the animal or plant, yet the wonderful symmetry of distribution, and the gradual shading of the colors is often far more regular than appears to be required. This result also will no doubt be ascribed to 'the laws of growth' but if we once admit such a principle at work, why bring in any other outside law to explain the perfection of those characters that are useful to the organism? If when a new species appears its colors happen to be so distributed that the individual gains some advantage, so much the better! If the color does not count one way or the other, then it does not enter into the problem of survival; but if it exposes an animal to a greater risk than it can surmount, that species will fail to hold its own. The same regularity and graduation of color exists also in animals that are microscopic, and no one thinks of accounting for these conditions through selection. Why then do we need a special explanation when the animals are so large that they attract our attention?

A number of writers, of whom Eimer is perhaps the most prominent, have insisted that evolution progresses along certain definite lines that are quite independent of selection of individual variations. The process has been called orthogenesis. Certain aspects of this view are in full accord with the theory of the survival of definite variations; for, we find, in fact, one of the most characteristic features in the appearance of definite variations to be that the same forms appear over and over again, showing a definite tendency to vary in certain definite directions. A striking case of this sort is that of the japanned peacocks described by Darwin, and of the mutations of the evening primrose described by de Vries. If future work can show that a change in a given direction is likely to be followed by others in the same direction, amongst some at least of the offspring, the process will have much in common with the process of orthogenesis. If this process should be in the direction of making some particular organ more perfect than the conditions demand, the new type may persist along with the parent form also, which it need not replace. If, on the contrary, the new acquirement unfits the new species for its environment the new species will never become established. Cases of this sort, in which a species continually gives off mutations, that can not survive, and yet continue to appear, are known.

In most cases the survival of a species is not determined by one particular character, but by the sum total of all. Therefore since the characters mutate independently, we might expect to find occasionally in a new species some characters more perfect than the actual requirements demand; other characters less perfect than is necessary to maintain the species if survival depended on these alone, and many characters that suffice for the demands of survival. Thus in man, to take but a single example, the ear and the eye may be more perfectly developed in some respects than the demands of survival require, while the appendix vermiformis is actually injurious to the welfare of the race. The majority of the peculiarities lie somewhere between these extremes. A new race of men can not be produced by selection of those individuals that show fluctuating variations in these different directions, but must arise by the sudden appearance of a new type or types, which, finding a foothold, may establish themselves along side of the present races.[1] It may be that definite variations are even at present occurring, but are not of sufficient importance or difference to attract attention. Permanent improvement must be looked for in this direction, and not from the picking out of those individuals that fluctuate in an advantageous direction.

How Adaptation may arise through the Appearance of Definite Variations.

If we take the position that the creation of new forms is not the outcome of a long continued process of remodeling of each species, but that new forms appear 'spontaneously,' how can we account for the adaptation of new species to their environment? Are we to believe that all new forms that appear from 'inner causes' will be already adapted to some external set of conditions? A very slight familiarity with living things will give, I believe, a negative answer to this question. Nevertheless let us not conclude too quickly that none of the new forms will be adapted to some environment, even if some of them are not.

Darwin defined natural selection as follows: 'This preservation of favorable individual differences and variations, and the destruction of those which are injurious, I have called natural selection, or the survival of the fittest.' He constantly compares the action of natural selection to that of artificial selection, in which he supposes that the breeder picks out those individual differences of the kind known as 'fluctuating variations.'

Modern zoologists who claim that the Darwinian theory is sufficiently broad to include the idea of the survival of definite variations seem inclined to forget that Darwin examined this possibility and rejected it. The grounds for this rejection seemed valid at the time, but a wider knowledge of the facts has shown that the problem is simpler than Darwin was aware of.

While Darwin uses the term 'struggle for existence' in a very loose, and often in a metaphorical sense, as he himself points out; and while it is true that he speaks of varieties and even species struggling with each other, yet the central idea is that natural selection adapts the organism to its environment by picking out and accumulating slight, individual differences. It is, indeed, only in this way that natural selection appears in the rôle of a creative factor in evolution, and it is this power to build up new adapted types that appears to give the theory its high value.

Numerous cases of discontinuous variation have been known for a long time, so that it is no mere assumption that such occur in nature. Darwin himself has collected many instances of this sort, and amongst domesticated animals and plants sudden variations have been frequently recorded. In fact there can be no doubt, especially in the case of plants, that such variations have often been utilized by the breeder, even unconsciously at times. A few cases of this sort have been described even for domesticated pigeons, and it is not improbable that the great variety of domesticated breeds may, in part, have arisen in this way, and not as the result of the selection of the individual fluctuations of the wild rock-pigeon. Darwin argued that sudden variations can not have been the source of new species, because, as a rule, when they are crossed with the parent form the offspring are not intermediate, but are exactly like one or the other parent, and it is a well known fact that when wild species are crossed the hybrid is midway in character between its parents. Therefore, Darwin believed, wild species could not have arisen as discontinuous variations.

Our knowledge on these points has greatly increased since Darwin's time, especially in recent years, mainly as the result of de Vries's work on the evening primrose. It appears that there are several different kinds of definite variations. The simplest cases are those in which some one character suddenly becomes lost, as when an albino mouse arises from a gray mouse. If such an albino is crossed with the parent gray form the offspring in the first generation are all like the gray mouse, but if these offspring are then inbred, they will produce some pure white which breed pure white, and some pure gray mice which breed pure gray, besides some impure forms like those of the first generation, which breed both colors. In other cases the new form if bred to the parent stock gives rise to offspring which even in the first generation are like the new form. Thus it appears that when the ancon ram was bred to an ordinary ewe the offspring were like the father. Similar cases are recorded for the merino sheep, for the japanned peacock, and for other new types. It appears here that the new type dominates in some or in all of the offspring. Darwin had in mind cases like these when he rejected the view that discontinuous variations have furnished the material for natural selection.[2]

There are, however, still other kinds of sudden and definite variations, and these, not the former, are the kind which are mainly responsible for new species. New elementary species arise not through a loss of some character of the old form (although new species may have amongst their new characters some that are due to loss), but by the appearance of a new character or characters which follow a different law of inheritance. When such elementary species are crossed with the parent type, a new type may arise, which is usually intermediate in character between the parents, but in some cases may be more like one than the other. Similar results follow when the new elementary species are crossed with each other. Thus the results are comparable to those that occur when Linnæan species are crossed, although in the case when forms belonging to two widely different Linnæan species are combined the results may be so complicated that it is beyond our power to give a satisfactory analysis of the results at present.[3]

The term mutation has been used for the sudden appearance of new variations belonging to any one of the preceding cases. Whether it may not prove necessary to use different terms for the different kinds of discontinuous variations in so far as they follow different laws of inheritance remains for the future to decide. We might, perhaps, use the term mutation (and mutant) only where new elementary species are formed; retrogression where a character is lost (elementary varieties being formed, which follow the Mendelian law of inheritance, as in the case of albinos, etc.); atavism when a latent character becomes active, as when a lost character reappears; and saltation where a new combination of characters is produced and where the new form being crossed with the parent-stock does not give the Mendelian ratio, as in the case of the ancon ram, the japanned peacock, etc., if, in fact, these cases do not really follow the same rule of inheritance as do the elementary varieties.

De Vries has pointed out that each step, each mutation, may not have been any greater than the difference between the extremes of fluctuating variations, and if this is the case we see that evolution may have been a very gradual process, although not necessarily a very slow one. Darwin's idea that the process of evolution was very gradual is in full harmony with the mutation theory, but on the latter view we can better understand how evolution may at times have been relatively rapid, and that no such enormous periods are required for the process, as the Darwinian school is inclined to assume.

The time has come, I think, when we are beginning to see the process of evolution in a new light. Nature makes new species outright. Amongst these new species there will be some that manage to find a place where they may continue to exist. How well they are suited to such places will be shown, in one respect, by the number of individuals that they can bring to maturity. Some of the new forms may be well adapted to certain localities, and will flourish there; others may eke out a precarious existence, because they do not find a place to which they are well suited, and can not better adapt themselves to the conditions under which they live; and there will be others that can find no place at all in which they can develop, and will not even be able to make a start. From this point of view the process of evolution appears in a more kindly light than when we imagine that success is only attained through the destruction of all rivals. The process appears not so much the result of the destruction of vast numbers of individuals, for the poorly adapted will not be able to make even a beginning. Evolution is not a war of all against all, but it is largely a creation of new types for the unoccupied, or poorly occupied places in nature.


In the preceding pages I have tried to bring into contrast the point of view of the Darwinian school and the newer conception of the survival of elementary species. I have tried to show what selection has meant to the selectionist. They have never hesitated to take each particular character of an animal or plant, and dress it up in more perfect garments, while the body of the species, if I may so speak, has been left as it was before. There has been a continual tampering with the characters of the organism with the laudable intention of doing with them that which nature herself seems unable to do, namely, to dissociate them from the rest of the organization and perfect them in this way or in that. It is this meddling with the fluctuating characters of the species that has been the characteristic procedure of the Darwinians, in their attempt to show how new species have been created. In contrast to this method, the theory of the survival of species assumes that a form once made does not have its individual parts later disassociated and adjusted to better fit the external needs of the species. Such a new form can change only by becoming again a new species with a new combination of characters; some of which may be more developed in one direction than before, others less; etc.

New forms on the Darwinian theory are supposed to be created by a process of picking out of individual differences. If, in addition to this, Darwin supposed that at times varieties and species crowd each other out nothing new is thereby created.[4] On the other hand the theory of the survival of definite variations refers the creation of new forms to another process, namely, to a sudden change in the character of the germ. The creating has already taken place before the question of the survival of the new form comes up. After the new form has appeared the question of its persistence will depend on whether it can get a foothold. The result is now the same as when species crowd each other out. This distinction appears to me to be not a matter of secondary interest, but one of fundamental importance, for it involves the whole question of the 'origin of species.' So far as a phrase may sum up the difference, it appears that new species are born; they are not made by Darwinian methods, and the theory of natural selection has nothing to do with the origin of species, but with the survival of already formed species. Not selection of the fittest individuals, but the survival of the sufficiently fit species.

There is a fundamental difference between the idea that fluctuating variations become specific characters through accumulation by selection, and the idea that new species arise as definite variations, which, with their appearance, characterize the new form as a new species. According to the Darwinian theory, natural selection performs a double duty, first, to build up new species, and, second, to maintain them in competition with other species. According to the other view, species are not formed by any kind of selection, and the question of survival only concerns the maintenance of species, already formed. The primary problem is the problem of the 'origin of species.' The central idea is not what species survive, but how species originate; no matter whether they are going to become victorious or not.

After a species has appeared it will surely be admitted by every one, that forms that can survive will survive! If Darwin's theory meant only this to those who adopted it, is it not surprising that such a truism should have been hailed as a great discovery? Was not the theory heralded because it seemed to explain how new species arose? What shall we say then when we find a situation like that existing at the present time, when we are told that after all the only difference between Darwin's theory of natural selection and the theory of the survival of definite variations is that in the one case fluctuating variations are selected, and in the other mutations, and that in both cases natural selection is the key to the evolutionary process! Is not the 'origin of species' still the real point at issue?

I yield to no one in admiration for what Darwin has done in behalf of the biological sciences, for he succeeded, where the great French zoologists failed, in establishing the principle of evolution. Furthermore no other hypothesis, that has as yet been proposed, accounts so well for the widespread occurrence of adaptation of organisms to the environment as does the principle of natural selection. But appreciation of Darwin's claims in these directions need not blind us to the insufficiency of the theory of natural selection to account for the origin of species; nor to the fact that his followers have been especially concerned in propounding and making application of this side of the theory. They have shown little interest in selection as the great conserving factor of evolution, and the reason for this is not far to seek, because of the much greater importance that they have attributed to natural selection as a creative factor in building up individual differences into specific characters.

  1. I do not, of course, mean to imply that any one of the present races of mankind could not be greatly improved artificially by encouraging the individuals best suited to civilized conditions to propagate, and by disencouraging propagation by the criminal, indolent and unhealthy individuals. Until this is done we can not hope for even an artificial improvement in the standard of the race.
  2. It is not clear from the records whether this class follows the Mendelian rule of inheritance, with the new form as the dominant in the first generation. From the statements that are ordinarily made, it might appear that even in the first generation some of the offspring are like one parent and some like the other, which, if true, would make these cases different from the first class given that follow Mendel's law of inheritance.
  3. In my book on 'Evolution and Adaptation,' I pointed out that while a single step or imitation could not he halved, yet if a number of steps were taken in the same direction the number might then be halved. This may be true but is unnecessarily hypothetical, and, as pointed out here in the text, the apparent difficulty that Darwin met with can now be explained in other ways.
  4. If the survival of certain species determines, in a metaphorical sense, the kinds of future mutations that occur, the course of evolution may appear to be guided by selection or survival; but however true it may be that selection acts by lopping off certain branches, and limits to this entent the kinds of possible future mutations, the origin of the new forms remains still a different question from the question of the survival of certain species. This negative action of selection is not the process that most Darwinians have had in mind as the source of the origin of new species. It is true that Weismann believes that selection of individual differences determines the origin of new species, and that the creation of these new species determines the future course of variations in the same direction, but his argument that fluctuating variations can go on indefinitely varying in the direction of selection is refuted by what has been actually found to be the case when the process of selection of fluctuating variations is carried out. Most of the individuals of a species may be brought in this way to show the particular character selected in its highest degree as a fluctuating variation, but it appears not possible to transgress this limit; and rigorous selection in every generation is necessary to hold the individuals to the highest point reached. Only by the appearance of new definite variations can a given character be permanently fixed, or a new species created that will show fluctuating variations around the new standard.