Numerical Effects of Natural Selection Acting upon Mendelian Characters.

The relation of recent work in genetics to the Darwinian theory is still in dispute. The value of the natural selection hypothesis can only be determined by appropriate experimental tests. At present there appears to be no inherent incompatibility between DARWIN’S fundamental principles and the newer concepts of pure lines, mutations, and unit characters. To some the action of natural selection seems to furnish an explanation of the actual trend of organic evolution which is lacking in later theories. The “pure line” concept does not assist materially in explaining the existing diversity of species. If a pure line remain unchanged indefinitely it is difficult to account for the host of pure lines that exist at present. If, on the other hand, a real variation can occur in a pure line now and again, natural selection would help to explain the trend toward adaptive characters which these lines exhibit. The “mutation” concept may represent the actual nature of variation better than the Darwinian notion of small fluctuating changes. Yet this scarcely affects DARWIN’S main contentions. The magnitude and frequency of the variations are relatively unimportant, since in any case such variations will accumulate in the course of generations. Their adaptiveness, however, is highly significant for biology, and the question remains whether adaptive variations can sort themselves out from nonadaptive by the mere working of natural selection, without the action of a vital force or entelechy. Finally, the Mendelian concept of “unit characters” in no way precludes the action of natural selection. Indeed it would seem to render easier the submission of the natural selection hypothesis to a crucial experimental test. I t should be noted, however, that the method generally employed in genetic work is not “natural” but “artificial” selection. Experimental-