Evolution and Adaptation

by Thomas Hunt Morgan

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 Variation

Galton, 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 specimens. But it does not at all follow because these intermediate forms have been found to exist, that they were the very stages that were passed through in the course of evolution. Counter evidence exists in abundance, not only of the appearance of considerable sports, but of their remarkable stability in hereditary transmission.” Comparing such an apparently continuous series with machines, Galton concludes, “If, however, all the variations of any machine that had ever been invented were selected and arranged in a museum, each would differ so little from its neighbors as to suggest the fallacious inference that the successive inventions of that machine had progressed by means of a very large number of hardly discernible steps.”

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 organized nature arranged in a discontinuous series of groups differing from each other by differences which are Specific; on the other hand, we see the diverse environments to which these forms are subject passing insensibly into each other. We must admit, then, that if the steps by which the diverse forms of life have varied from each other have been insensible,—if, in fact, the forms ever made up a continuous series,—these forms cannot have been broken into a discontinuous series of groups by a continuous environment, whether acting directly as Lamarck would have, or as selective agent as Darwin would have. This supposition has been generally made and admitted, but in the absence of evidence as to Variation it is nevertheless a gratuitous assumption, and, as a matter of fact, when the evidence as to Variation is studied, it will be found to be in a great measure unfounded.”

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. He then makes the following significant statement: “All the characters above enumerated which are transmitted in a perfect state to some of the offspring and not to others—such as distinct colors, nakedness of skin, smoothness of leaves, absence of horns or tail, additional toes, pelorism, dwarfed structure, etc., have all been known to appear suddenly in individual animals or plants. From this fact, and from the several slight, aggregated differences which distinguish domestic races and species from each other, not being liable to this peculiar form of transmission, we may conclude that it is in some way connected with the sudden appearance of the characters in question.”

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.