Biology and Its Makers / With Portraits and Other Illustrations

by William A. Locy

Variation.—It will be noticed that two of the causes assigned by Darwin are the same as those designated by Lamarck, but their treatment is quite different. Darwin (Fig. 113) assumed variation among animals and plants without attempting to account for it, while Lamarck undertook to state the particular influences which produce variation, and although we must admit that Lamarck was not entirely successful in this attempt, the fact that he undertook the task places his contribution at the outset on a very high plane.

Fig. 113.—Charles Darwin, 1809-1882.

The existence of variation as established by observation is unquestioned. No two living organisms are exactly alike at the time of their birth, and even if they are brought up together under identical surroundings they vary. The variation of plants and animals under domestication is so conspicuous and well known that this kind of variation was the first to attract attention. It was asserted that these variations were perpetuated because the forms had been protected by man, and it was doubted that animals varied to any considerable extent in a state of nature. Extended collections and observations in field and forest have, however, set this question at rest.

If crows or robins or other birds are collected on an extensive scale, the variability of the same species will be evident. Many examples show that the so-called species differ greatly in widely separated geographical areas, but collections from the intermediate territory demonstrate that the variations are connected by a series of fine gradations. If, for illustration, one should pass across the United States from the Atlantic to the Pacific coast, collecting one species of bird, the entire collection would exhibit wide variations, but the extremes would be connected by intermediate forms.

The amount of variation in a state of nature is much greater than was at first supposed, because extensive collections were lacking, but the existence of wide variation is now established on the basis of observation. This fact of variation among animals and plants in the state of nature is unchallenged, and affords a good point to start from in considering Darwinism.

Inheritance.—The idea that these variations are inherited is the second point. But what particular variations will be preserved and fostered by inheritance, and on what principle they will be selected, is another question—and a notable one. Darwin's reply was that those variations which are of advantage to the individual will be the particular ones selected by nature for inheritance. While Darwin implies the inheritance of acquired characteristics, his theory of heredity was widely different from that of Lamarck. Darwin's theory of heredity, designated the provisional theory of pangenesis, has been already considered (see Chapter XIV).

Natural Selection.—Since natural selection is the main feature of Darwin's doctrine, we must devote more time to it. Darwin frequently complained that very few of his critics took the trouble to find out what he meant by the term natural selection. A few illustrations will make his meaning clear. Let us first think of artificial selection as it is applied by breeders of cattle, fanciers of pigeons and of other fowls, etc. It is well known that by selecting particular variations in animals and plants, even when the variations are slight, the breeder or the horticulturalist will be able in a short time to produce new races of organic forms. This artificial selection on the part of man has given rise to the various breeds of dogs, the 150 different kinds of pigeons, etc., all of which breed true. The critical question is, Have these all an individual ancestral form in nature? Observation shows that many different kinds—as pigeons—may be traced back to a single ancestral form, and thus the doctrine of the fixity of species is overthrown.

Now, since it is demonstrated by observation that variations occur, if there be a selective principle at work in nature, effects similar to those caused by artificial selection will be produced. The selection by nature of the forms fittest to survive is what Darwin meant by natural selection. We can never understand the application, however, unless we take into account the fact that while animals tend to multiply in geometrical progression, as a matter of fact the number of any one kind remains practically constant. Although the face of nature seems undisturbed, there is nevertheless a struggle for existence among all animals.