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A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED

We use the word evolution in many ways—to include many different kinds of changes. There is hardly any other scientific term that is used so carelessly—to imply so much, to mean so little.

Three Kinds of Evolution

We speak of the evolution of the stars, of the evolution of the horse, of the evolution of the steam engine, as though they were all part of the same process. What have they in common? Only this, that each concerns itself with the history of something. When the astronomer thinks of the evolution of the earth, the moon, the sun and the stars, he has a picture of diffuse matter that has slowly condensed. With condensation came heat; with heat, action and reaction within the mass until the chemical substances that we know today were produced. This is the nebular hypothesis of the astronomer. The astronomer explains, or tries to explain, how this evolution took place, by an appeal to the physical processes that have been worked out in the laboratory, processes which he thinks have existed through all the eons during which this evolution was going on and which were its immediate causes.

When the biologist thinks of the evolution of animals and plants, a different picture presents itself. He thinks of series of animals that have lived in the past, whose bones (fig. 1) and shells have been preserved in the rocks. He thinks of these animals as having in the past given birth, through an unbroken succession of individuals, to the living inhabitants of the earth today. He thinks that the old, simpler types of the past have in part changed over into the more complex forms of today.

He is thinking as the historian thinks, but he sometimes gets confused and thinks that he is explaining evolution when he is only describing it.

Fig. 1. A series of skulls and feet. Eohippus, Mesohippus, Meryhippus, Hipparion and Equus. (American Museum of Natural History. After Matthews.)

A third kind of evolution is one for which man himself is responsible, in the sense that he has brought it about, often with a definite end in view.

His mind has worked slowly from stage to stage. We can often trace the history of the stages through which his psychic processes have passed. The evolution of the steam-boat, the steam engine, paintings, clothing, instruments of agriculture, of manufacture, or of warfare (fig. 2) illustrates the history of human progress. There is an obvious and striking similarity between the evolution of man's inventions and the evolution of the shells of molluscs and of the bones of mammals, yet in neither case does a knowledge of the order in which these things arose explain them. If we appeal to the psychologist he will probably tell us that human inventions are either the result of happy accidents, that have led to an unforeseen, but discovered use; or else the use of the invention was foreseen. It is to the latter process more especially that the idea of purpose is applied. When we come to review the four great lines of evolutionary thought we shall see that this human idea of purpose recurs in many forms, suggesting that man has often tried to explain how organic evolution has taken place by an appeal to the method which he believes he makes use of himself in the inorganic world.

Fig. 2. Evolution of pole arms. (Metropolitan Museum. After Dean.)

What has the evolution of the stars, of the horse and of human inventions in common? Only this, that in each case from a simple beginning through a series of changes something more complex, or at least different, has come into being. To lump all these kinds of changes into one and call them evolution is no more than asserting that you believe in consecutive series of events (which is history) causally connected (which is science); that is, that you believe in history and that you believe in science. But let us not forget that we may have complete faith in both without thereby offering any explanation of either. It is the business of science to find out specifically what kinds of events were involved when the stars evolved in the sky, when the horse evolved on the earth, and the steam engine was evolved from the mind of man.

Is it not rather an empty generalization to say that any kind of change is a process of evolution? At most it means little more than that you want to intimate that miraculous intervention is not necessary to account for such kinds of histories.

We are concerned here more particularly with the biologists' ideas of evolution. My intention is to review the evidence on which the old theory rested its case, in the light of some of the newer evidence of recent years.

Four great branches of study have furnished the evidence of organic evolution. They are:

Comparative anatomy.

Embryology.

Paleontology.

Experimental Breeding or Genetics.

The Evidence from Comparative Anatomy

When we study animals and plants we find that they can be arranged in groups according to their resemblances. This is the basis of comparative anatomy, which is only an accurate study of facts that are superficially obvious to everyone.

The groups are based not on a single difference, but on a very large number of resemblances. Let us take for example the group of vertebrates.

Fig. 3. Limb skeletons of extinct and living animals, showing the homologous bones: 1, salamander; 2, frog; 3, turtle; 4, Aetosaurus; 5, Pleisiosaurus; 6, Ichthyosaurus; 7, Mesosaurus; 8, duck. (After Jordan and Kellogg.)

The hand and the arm of man are similar to the hand and arm of the ape. We find the same plan in the forefoot of the rat, the elephant, the horse and the opossum. We can identify the same parts in the forefoot of the lizard, the frog (fig. 3), and even, though less certainly, in the pectoral fins of fishes. Comparison does not end here. We find similarities in the skull and back bones of these same animals; in the brain; in the digestive system; in the heart and blood vessels; in the muscles.

Each of these systems is very complex, but the same general arrangement is found in all. Anyone familiar with the evidence will, I think, probably reach the conclusion either that these animals have been created on some preconceived plan, or else that they have some other bond that unites them; for we find it difficult to believe that such complex, yet similar things could have arisen independently. But we try to convince our students of the truth of the theory of evolution not so much by calling their attention to this relation as by tracing each organ from a simple to a complex structure.

I have never known such a course to fail in its intention. In fact, I know that the student often becomes so thoroughly convinced that he resents any such attempt as that which I am about to make to point out that the evidence for his conviction is not above criticism.

Fig. 4. Drosophila ampelophila. a, Female and b, male.

Because we can often arrange the series of structures in a line extending from the very simple to the more complex, we are apt to become unduly impressed by this fact and conclude that if we found the complete series we should find all the intermediate steps and that they have arisen in the order of their complexity. This conclusion is not necessarily correct. Let me give some examples that have come under my own observation. We have bred for five years the wild fruit fly Drosophila ampelophila (fig. 4) and we have found over a hundred and twenty-five new types that breed true. Each has arisen independently and suddenly. Every part of the body has been affected by one or another of these mutations. For instance many different kinds of changes have taken place in the wings and several of these involve the size of the wings. If we arrange the latter arbitrarily in the order of their size there will be an almost complete series beginning with the normal wings and ending with those of apterous flies. Several of these types are represented in figure 5. The order in which these mutations occurred bears no relation to their size; each originated independently from the wild type.

Fig. 5. Mutants of Drosophila ampelophila arranged in order of size of wings: (a) cut; (b) beaded; (c) stumpy; (d) another individual of stumpy; (f) vestigial (g) apterous.

The wings of the wild fly are straight (fig. 4). Several types have arisen in which the wings are bent upwards and in the most extreme type the wings are curled over the back, as seen in figure 54 (g), yet there is no historical connection between these stages.

Mutations have occurred involving the pigmentation of the body and wings. The head and thorax of the wild Drosophila ampelophila are grayish yellow, the abdomen is banded with yellow and black, and the wings are gray. There have appeared in our cultures several kinds of darker types ranging to almost black flies (fig. 20) and to lighter types that are quite yellow. If put in line a series may be made from the darkest flies at one end to the light yellow flies at the other. These types, with the fluctuations that occur within each type, furnish a complete series of gradations; yet historically they have arisen independently of each other.

Many changes in eye color have appeared. As many as thirty or more races differing in eye color are now maintained in our cultures. Some of them are so similar that they can scarcely be separated from each other. It is easily possible beginning with the darkest eye color, sepia, which is deep brown, to pick out a perfectly graded series ending with pure white eyes. But such a serial arrangement would give a totally false idea of the way the different types have arisen; and any conclusion based on the existence of such a series might very well be entirely erroneous, for the fact that such a series exists bears no relation to the order in which its members have appeared.

Suppose that evolution "in the open" had taken place in the same way, by means of discontinuous variation. What value then would the evidence from comparative anatomy have in so far as it is based on a continuous series of variants of any organ?

No one familiar with the entire evidence will doubt for a moment that these 125 races of Drosophila ampelophila belong to the same species and have had a common origin, for while they may differ mainly in one thing they are extremely alike in a hundred other things, and in the general relation of the parts to each other.

It is in this sense that the evidence from comparative anatomy can be used I think as an argument for evolution. It is the resemblances that the animals or plants in any group have in common that is the basis for such a conclusion; it is not because we can arrange in a continuous series any particular variations. In other words, our inference concerning the common descent of two or more species is based on the totality of such resemblances that still remain in large part after each change has taken place. In this sense the argument from comparative anatomy, while not a demonstration, carries with it, I think, a high degree of probability.

The Evidence from Embryology

In passing from the egg to the adult the individual goes through a series of changes. In the course of this development we see not only the beginnings of the organs that gradually enlarge and change into those of the adult animal, but also see that organs appear and later disappear before the adult stage is reached. We find, moreover, that the young sometimes resemble in a most striking way the adult stage of groups that we place lower in the scale of evolution.

Many years before Darwin advanced his theory of evolution through natural selection, the resemblance of the young of higher animals to the adults of lower animals had attracted the attention of zoölogists and various views, often very naïve, had been advanced to account for the resemblance. Among these speculations there was one practically identical with that adopted by Darwin and the post-Darwinians, namely that the higher animals repeat in their development the adult stages of lower animals. Later this view became one of the cornerstones of the theory of organic evolution. It reached its climax in the writings of Haeckel, and I think I may add without exaggeration that for twenty-five years it furnished the chief inspiration of the school of descriptive embryology. Today it is taught in practically all textbooks of biology. Haeckel called this interpretation the Biogenetic Law.