Evolution: Its nature, its evidence, and its relation to religious thought

by Joseph LeConte

Now, if art can vary form so greatly, and in so short time, why may not Nature in limitless time? If art by artificial selection, why not Nature by natural selection? Nature is as rigid in selection and as ruthless in destruction: why may we not expect similar or even much greater results? The process is similar in the two cases—i. e., selection among varieties in offspring, only that the selection is natural instead of artificial, and the process is so slow that there is little tendency to reversion in the latter case. Suppose, then, we have a gradually changing physical environment, or climate. Among the divergent varieties of any species in each generation, those would be preserved which are most in accordance with the new climate, and the others would perish. This is natural selection, or survival of the fittest. Add to this the effect of the change in the organic environment. All species are modified by the changing physical environment; but these modified species again all affect one another in the competitive struggle for life, and the strongest or swiftest, or most cunning, survive (natural selection). Add to this, again, the struggle among the males for possession of the females—for reproductive opportunities—by which only the strongest and most courageous, or the most beautiful and attractive, leave progeny which inherit their peculiarities (sexual selection). Add to these, finally, migrations, voluntary among higher and involuntary dispersals among lower animals and plants, and the consequent mingling of faunas and floras—the migrations subjecting them to great change of environment, both physical and organic, and the mingling producing fiercer struggle for life—and we have in powerful operation many causes of modification. Add, I say, all these causes of modification together, and then make the process slow and continuous through unlimited time, and where is the limit to the degree of change? Commencing in any species, from any point of departure, there are formed first slight modifications which would be called varieties; then these modifications, continuing in the same direction, form races; these races by wider separation become species, and species in their turn become genera, etc. Comparing, again, to a growing tree, varieties are swelling buds; when they grow into twigs, they are species; when they branch again into different species, the branching stem becomes a genus, etc.

We have thus far spoken only of the various forms of one factor, viz., the Darwinian factor of selection, whether natural or artificial. We have dwelt upon this one, because the natural and the artificial processes are so similar, and the artificial is so controllable. But there are other factors in operation, in art as well as in nature. We have already spoken ([p. 73]) of other factors of natural change. We have shown how changing physical environment affects function, and function affects form and structure, and how these slight changes are integrated by heredity through many generations. We have also shown how use or disuse increases or diminishes the size and change the form of parts, and these changes, also, however slight, are integrated by heredity.

Now, these factors are operative also in domestication of animals and cultivation of plants. No environment is so new and peculiar as domestication and cultivation. The soil and temperature in plants, food and housing of domesticated animals, tend to change form and structure of the offspring, although in a way which it is difficult intelligently to control, and thus are prolific of varieties from which to select. In fact, they often give rise to great and unexpected modifications, called sports, which form points of departure for new varieties and races. Now, in nature, not only are all these causes and factors of change in constant operation, but they act together in a peculiarly complex way. All the members of a fauna and flora, and the physical environment of any locality, constitute together a most complex and delicately adjusted system of correlated parts. A change in one part is propagated through the whole system; also, a change in one factor affects all other factors. When we add to this the large amount of time, in comparison with individual human life and observation, necessary to produce visible change of form, we can easily understand why the process is still imperfectly understood, although the fact is certain.

But it will be asked, Are there, then, no differences between the artificially made extreme varieties equivalent, so far as difference of form is concerned, to species, and real natural species? There are. If there were not, there would never have been any doubt about the derivative origin of natural species. But if it be asked, Are not these differences fundamental, and therefore fatal to the argument for evolution derived from this source? we answer, we think not. We will deal frankly and fairly with these differences.

First Difference, Reversion.—The strong tendency of artificial varieties to reversion, even during the process of formation, and especially their complete reversion to the original type if the hand of man be withdrawn—i. e., if left to themselves, or become wild—is supposed to show an essential difference between such varieties, however extreme, and true species—is supposed, in fact, to prove an indestructible permanency of specific types. Nature disowns these artificial forms, and as it were brands them with bastardy. Not only so, she strives ever to destroy them. The supporting hand of man is necessary to sustain them. Left to themselves and to Nature, they quickly revert to the original type. If all the extreme varieties of dogs, from the greyhound and Newfoundland, on the one hand, to the terrier and lap-dog on the other, were turned loose on an isolated island, uninhabited by man but full of other animals, and left there to shift for themselves—and the island were visited again after a lapse of a hundred or a thousand years—it is probable that a uniform species, something like to, though perhaps not identical with, the wolf, would be found. They would have reverted to the original or nearly the original wild type from which they were produced by domestication. All or nearly all that was done by man would have been undone by Nature. This reversion is one test of species.

But the reason of this tendency to reversion is obvious: First, the time was too short, the rate of change was too rapid, in the artificial formation of these varieties. There was not time enough to accumulate a fund of heredity on each successive stage of the change. Therefore the form is unstable and the tendency to revert is strong. Compare the fleeting days and the hurrying impatience of man with the infinite time and the divine patience of Nature! But mere instability is not the principal cause of reversion. Secondly, in the case of artificial forms in a wild state, natural selection compels reversion. Every species in a wild state must of course be in harmony with the environment. But artificially made forms are in harmony with the artificial environment of domestication, but not with the environment of nature. In nature the fittest survive, but artificial breeds are not fit to survive in a state of nature. They are therefore quickly destroyed in the struggle for life, or must be modified. Nature immediately begins to select the fittest, and gradually in the course of time produces one or more uniform species, similar to that from which they came, or perhaps to what they would have been by this time if left to the operation of natural causes under the conditions supposed. But natural species, if they are formed, as the derivationists suppose, by the operation of natural causes, can not revert unless the conditions revert; for the same causes which operated to produce, still continue to operate to keep, the species. Take an example:

The form, the habits, and the instincts of the pointer have been made by a slow process of artificial selection of divergent varieties of offspring, and by training of individuals continued and its effects accumulated through many generations. But this form and these habits and instincts, so laboriously produced, would be quickly destroyed by Nature. The pointer, left to himself, must either change or become extinct, because not adapted to the wild state. Such instincts and habits would not only be of no use, but would be incompatible with success in the struggle for life. But suppose for a moment that these habits and instincts were useful to the animal in a wild state; evidently they would be instantly seized upon by natural selection, and not only perpetuated but intensified until a very distinct species would be produced. The same is true of all other races of dogs. If the Newfoundland, the greyhound, and the pug were all turned loose in a forest, and if each of these kinds were admirably adapted to some place in the economy of Nature—for some special mode of food-getting without corresponding disabilities in other directions (as must be the case if made by natural selection)—there can be no doubt they would each survive, and their characters intensified; intermediate forms would disappear (for reasons which we shall see presently), and we would soon have three distinct species, or perhaps we would even call them distinct genera.

Second Difference, Intermediate Forms.—Natural species are distinct—marked out with hard and fast lines—while artificially-made races, even though in their typical forms they differ as much or more than natural species, shade into one another by insensible gradations. In answer and explanation of this difference we remark: If species or modified forms of any kind, whether natural or artificial, are made by natural causes, and not at once out of hand by supernatural creation, then of course there must have been gradations in the process of making. Now, in the artificial case, the whole process as well as the result lies within the limits of observation, while in the natural case only the final result. But it will be asked, Why are the gradations not seen also in the final result? We answer, because the intermediate forms are eliminated in the struggle for life, and not reproduced by cross-breeding. If artificial races always bred true—i. e., without crossing, as natural species do—they would probably soon be as sharply demarked. Cross-breeding is the great cause of the shadings between domestic races. This brings me to the third and most important difference.

Third Difference, Cross-Fertility.—Artificially-made races breed freely and without repugnance with one another, and the offspring of such cross-breeding is indefinitely fertile. Natural species will not usually unite with one another, being prevented by sexual repugnance and other causes. Or, if they do sexually unite, there is either no offspring, or else the offspring is sterile, and therefore the intermediate form dies out in the first generation; or else the offspring is imperfectly fertile, and therefore the intermediate form is eliminated in a few generations, and the species remain distinct; or else the offspring is more fertile with the parent stocks, and therefore revert to the parent stocks, and still the species remain distinct. Such infertile, or imperfectly fertile, offspring—the result of crossing of species—are called hybrids.

This is regarded as a most important test of true species, as contrasted with varieties or races. There are two bases on which species may be founded. Species may be based on form, morphological species; or they may be based on reproductive functions, physiological species. By the one method a certain amount of difference of form, structure, and habit, constitutes species; according to the other, if the two kinds breed freely with each other and the offspring is indefinitely fertile, the kinds are called varieties, but if they do not they are called species. The two tests, however, do not always accord. Every now and then we find undoubted morphological species which may be crossed and produce indefinitely fertile offspring. Yet it is certainly true that species are usually cross-sterile, while varieties, whether natural or artificial, are cross-fertile.