A History of Evolution/Chapter 6
CHAPTER VI.
THE POST-DARWINIANS: DEVRIES AND THE
MUTATION THEORY.
The period between 1860 and 1900 was occupied largely by elaborations of the Darwinian conception of evolution, and arguments as to whether or not organic descent was a fact. In those four decades there were many famous workers—Alfred Russell Wallace, co-discoverer with Darwin of the theory of selection; Weismann and Haeckel, Germany's great evolutionists; the philosopher, Spencer; Cope, the American paleontologist, and Huxley, the English champion of scientific rationalism—these, and a host of others spent their lives in demonstrating the workings of evolution. But unfortunately, the opposition which they encountered forced them to write and work largely along lines of argument and thus much of their work was fruitless so far as the discovery of new principles is concerned.
During this same period the doctrine of evolution suffered much from over-enthusiasm on the part of some of its defenders. Even Wallace overdid the hypothesis of sexual selection, and the kindred hypotheses of concealing and protective coloration. Naturalists sought to explain every coloring of animals and plants as being of some value to them, and therefore the real cause of the existence of the species; not a few carried the idea of value in sexual differences, such as those between the male and female peacock, to a similar extreme. But in spite of the inaccuracies which they published, these enthusiasts did far more good than harm, for they aided greatly in securing popular support for the main theory.
It was toward the beginning of this century that evolutionary studies received another great stimulus. Professor Hugo de Vries, a Dutch botanist of considerable note, proposed what he called the "mutation theory" as a substitute for Darwin's conception of "natural selection." He began his studies by attempting to produce by careful selection a variety of buttercup which should contain in its flower more than the normal number of petals. He actually achieved the desired increase, but it was far from a stable condition; while some of the flowers possessed eight, nine, or ten petals, and a few as high as thirty-one, many of them possessed the original number, five. When selection was abandoned there appeared at once a general retrogression toward the primitive state, and this fact caused de Vries to conclude that selection alone was not enough to cause the formation of a new species of plant or animal[1]. Instead, he concluded that when a change of permanent value took place in a plant or animal it was something entirely different from the constant variations on which Darwin and his followers relied; it was a discontinuous variation—a 'sport,' the florist or gardener would call it—to which de Vries applied the new name mutation. Mutation, he believed, involved a very definite change in the reproductive cells of the organism—a change which had absolutely no relation to the environment. They arose from conditions within the plant and animal, and might or might not affect it favorably. Those mutations which were not beneficial would be eliminated by selection; those which were of value to the creature would probably be preserved. Thus, in de Vries' mind evolution was a process due primarily to internal causes, its course being merely guided by environment, which selected those mutations capable of surviving.
Without question, de Vries had a real basis for his theory. Mutations do take place among both wild and domestic creatures; thus among the dandelions there constantly appear special types which breed true and are, as Castle has called them, "little species within the dandelion species." Similar mutations are well known in peas, beans, evening primroses, and such domestic animals as the sheep. Clearly, therefore, species do arise as de Vries stated; the question is, is this the only way in which they arise?
This problem was raised little more than twenty years ago—a period far too short to allow for the settling of a question that is merely another statement of the problem that has puzzled scientists and philosophers for more than twenty centuries.
There is, however, excellent reason for believing that the conceptions of both de Vries and Darwin are true; that neither of them excludes the other from operation. Thus in the famous chalk formation of England there may be found an evolutionary chain of sea-urchina which, according to the general concensus of opinion, represent true Darwinian evolution. As N. C. Macnamara says, "They are first found in their shelled, sparsely ornamented forms, from which spring, as we ascend the zone, all the other species of the genus. The progression is unbroken and minute in the last degree. We can connect together into continuous series each minute variation and each species of graduation of structure so insensible that not a link in the chain of evidence is wanting."
On the other hand, the writer has recently completed a microscopic study of a group of ancient lamp-shells—animals which looked somewhat like molluscs, but which were very different internally—with altogether different results. The particular changes involved were minor matters of surface markings, which could have had no conceivable importance to the animals. Selection, therefore, may be virtually ruled out: indeed, many of the different forms lived close together, with apparently equal success. But in the small markings on the shells there appear, as one follows the series from bottom to top, very decided changes, and those changes are, in some cases, abrupt and complete.
In others the variations are very small—indeed they could be distinguished only with the microscope—but so far as could be told, were distinct. This, therefore, points to a course of evolution that was clearly a matter of mutation, without any apparent governing by the process of natural selection.
The conclusion which we may reach, therefore, is that both natural selection and mutation operate in the development of new forms from old. The variations, for which Darwin was at a complete loss to account, are in many cases the mutations emphasized by de Vries and his followers. But to what extent climate, food; habits, and multitudinous other environmental factors, coupled with such internal ones as racial old age, complicate the processes of variation and selection cannot yet be said. De Vries, in his mutation theory, supplied one of the deficiencies of Darwinism, and at the same time led scientists in general to realize that evolution is a far more complex problem than was supposed during the later portion of the last century. Darwin's primitive mudfish, with its trace of mind, and the process of natural selection, will not by any means account for the multitude of higher vertebrate forms which people, and have peopled the lands and waters of the globe.
At the same time the scientific public was awaking to the fact that evolution was an almost inconceivably complex affair, many of the post-Darwinian hypotheses began to show themselves of very doubtful importance. The theory of sexual selection, which Darwin elaborated in the "Descent of Man" began a steady decline. Such selection undoubtedly does take place, but it is not carried on to so great an extent as was once supposed. The idea of the protective value of colors and color arrangement, too, began to be doubted, although at the same time its principles became much better known and therefore more strongly emphasized by some naturalists. Inheritance of directly acquired characters was proved to be an impossibility, and much doubt was thrown upon the hypothesis of use and disuse. Instead of legs disappearing because they. are not used, they are now thought to disappear because the evolutionary processes going on within the animal demands their disappearance. What these processes are we do not know, but our frank avowal of ignorance gives us a certain confidence that we shall eventually find out.
But it is not only ideas that have changed within the last two decades; methods of study have undergone an even greater revolution. De Vries, at almost the same time he discovered mutation, rediscovered the fact that heredity was by no means so mysterious and erratic as it had been generally thought. Animals and plants, he discovered, possessed many characters which behaved in very definite ways when two varieties were crossed, and that the characters of an organism could be determined largely by the interbreeding of its ancestors. Thus arose the science of genetics, which seeks to find out the numerous factors underlying the various phenomena of heredity. And since heredity is the base of all evolution, genetics has for its ultimate aim the determination of the causes of that great process which is responsible for the existence of whatever animals and plants inhabit and have inhabited the earth. The geneticist is the most modern of evolutionists; he is not satisfied with finding out what has taken place in the past; he sets out to make evolution, or tiny portions of it, take place within his own laboratories and greenhouses.
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Today, despite the assertions of a few of its opponents, the theory of organic evolution is more thoroughly alive than it has ever been before. Paleontologists are studying their fossil shells and corals and bones in order to find out what has taken place during the millions upon millions of years during which living things have inhabited our planet. Anatomists are studying the bodies of modern animals, from the simplest to the highest, to determine their relationships one to the other; embryologists are tracing out the evolution of the individual in his life before birth. The geneticists are breeding plants, rabbits, mice, fishes, flies, potato bugs so that they may discover what evolution is doing today. Everywhere men are studying, comparing, experimenting. Their purpose is not to discover whether or not evolution is a fact; on that point they have long ago been satisfied. They are trying to find out how it operates and what forms it has produced; how differences arise among organisms, and what are their effects, and by what means they are passed from one generation to another until they become part and parcel of the inheritance, thereby establishing a new species.
- ↑ This conclusion was probably unjustified; his observation covered too short a period to mean a great deal.