Part 167. Selected adaptations. Dissatisfaction with Lamarck's explanation of modification through acquirement, deterred most of his contemporaries from accepting the theory of evolution, and it was not until Charles Darwin and Alfred Russel Wallace offered an explanation based upon acknowledged facts that evolutionism was welcomed at all widely by scientific thinkers. The new doctrine is generally known as Darwinism, because Darwin's contributions in support of it were most extensive and important, although both Darwin and Wallace, working quite independently, came to substantially the same conclusions, and announced them at the same meeting of the Linnaean Society of London in 1858.

Darwin felt that the problem of how one species changes into another might best be attacked by studying the modifications which plants and animals undergo when domesticated. We have already seen how much has been accomplished with cultivated plants by artificial selection,- farmers taking their seed from those individuals which please them best and continuing to pick out for propagation year after year the plants which show most fully those most desirable features. Here it would seem as if hereditary peculiarities were surely depended upon, and that the most striking departures from an original form represented the sum of many small differences in one direction accumulated through successive generations by inheritance. Darwin reasoned that if it could be shown that plants and animals in nature were continually subjected to agencies which favored the propagation of individuals possessed of slight hereditary peculiarities contributing to their welfare, the most perfect adaptations would be accounted for and the origin of species scientifically explained.

What natural agencies can be supposed to exert a selective power at all comparable to man's influence in developing artificial varieties? In the first place Darwin pointed to the fact that every kind of plant or animal produces more offspring than can possibly come to maturity; for if none died without issue, the progeny of one individual or pair of individuals would shortly cover the whole earth. Thus fifty dandelion plants from one, each bearing fifty-fold, and so on for nine generations, would make more than enough dandelions to occupy every foot of dry land on the globe. Innumerable individuals of innumerable kinds are continually producing offspring in large numbers. Since many more are produced than can mature it appears that a competitive struggle for life must be going on throughout the living world; and, he argued, so intense is this struggle that if some individuals had an adaptational peculiarity which gave them even a slight advantage over their rivals, such favored individuals would be the ones to survive and leave offspring. Some of their offspring would be sure (judging from breeders' experience) to inherit the same peculiarity, and these favored offspring would in turn transmit it more or less enhanced to certain of their descendants, and so on along lines of favored individuals in which the peculiarity in question would become more and more pronounced. This improvement would continue up to the point of adapting the organism as perfectly as possible to its environment. In this way a favored race might become a new species through survival of the individuals best fitted to a slowly changing or slightly changed environment.

The natural agencies which favor the survival of certain races in the struggle for existence Darwin compared in a metaphorical way to the human agencies which control the production of cultivated varieties. Both Man and Nature, he argued, act by selecting through successive generations certain peculiarities. Man chooses to encourage what pleases him. His method is Artificial Selection. Nature encourages only those peculiarities which best fit the organism to its environment; her way Darwin called Natural Selection. The phrase has been often misunderstood through failing to take it merely as a symbol of the way certain natural forces act.

Darwin defined natural selection as meaning "the preservation of favored races in the struggle for existence." He accepted Herbert Spencer's phrase, "survival of the fittest" as a good equivalent.

The theory of natural selection assumes, then, a struggle for existence among contemporary individuals in a given environment-a struggle resulting from vastly more of each kind being produced than can possibly survive, and so intense that even a slightly advantageous peculiarity may be enough to secure for its possessor both life and offspring. Such favoring peculiarities, according to the theory, are to be found in the small differences observable among individuals of the same kind living in the same environment. Hence we need not suppose them to be acquired, and we may safely regard them as hereditary because experience shows that peculiarities of this sort are regularly inherited. Since these slight departures from the parent form take various directions under the same circumstances, they are termed fluctuating variations, in distinction from definite variations, which are all in the same direction, as must be the case with those due to the same influence-like a direct effect of the environment-acting upon similarly constituted organisms. Darwin did not pretend to explain the origin of fluctuating variations further than to regard them as constitutional peculiarities resulting from the interplay of internal forces. These he saw might be so delicately balanced as to be readily disturbed by change of environment, and he recognized that a change of conditions commonly induces variability, and so gives a wider range of differences for selection to work upon, as when a horticulturist desiring to develop a new variety subjects his seedlings to unaccustomed conditions. But in so far as the variations are in different directions, they must be regarded as induced by the change rather than produced directly by it, and so may be spontaneous rather than acquired.