390 DARWINISM TO-DAY. 



ers' conception was fairly sufficient for practical purposes, but science 

 needed a clear understanding of the factors in the general process 

 of variation. Repeatedly Darwin tried to formulate these causes, 

 but the evidence available did not meet his requirements. 



"Quetelet's law of variation had not yet been published. Mendel's 

 claim of hereditary units, for the explanation of certain laws of 

 hybrids discovered by him, was not yet made. The clear distinction 

 between spontaneous and sudden changes, ars compared w r ith the 

 ever-present fluctuating variations, is only of late coming into recog- 

 nition by agriculturists. Innumerable minor points which go to elu- 

 cidate the breeders' experience, were unknown in Darwin's time. 

 No wonder that he made mistakes, and laid stress on modes of 

 descent which have since been proved to be of minor importance 

 or even of doubtful validity. 



"Notwithstanding all these apparently unsurmountable difficulties, 

 Darwin discovered the great principle which rules the evolution of 

 organisms. It is the principle of natural selection. It is the sifting 

 out of all organisms of minor \vorth through the struggle for life. 

 It is only a sieve, and not a force of nature, no direct cause of 

 improvement, as many of Darwin's adversaries, and unfortunately 

 many of his followers also, have so often asserted. It is only a 

 sieve, which decides which is to live, and what is to die. But evo- 

 lutionary lines are of great length, and the evolution of a flower, or 

 of an insectivorous plant is a way with many side-paths. It is the 

 sieve that keeps evolution on the main line, killing all, or nearly 

 all that try to go in other directions. By this means natural selec- 

 tion is the one directing cause of the broad lines of evolution. 



"Of course, with the single steps of evolution it has nothing to 

 do. Only after the step has been taken, the sieve acts, eliminating 

 the unfit. The problem, as to how the individual steps are brought 

 about, is quite another side of the question" (De Vries, "Species 

 and Varieties," pp. 4-7, 1905). 



The distinguished French zoologist (Professor in the University 

 of Paris), Delage, leader among French morphologists and experi- 

 Delage's esti- menters, voices his position concerning Darwinism in 

 mate of selection, the following concise phrases ("L'Heredite," 2d ed., 

 P- 397- 1903) ''La selection naturelle est un principe admirable et 

 parfaitement juste. Tout le monde est d'accord aujourd'hui sur ce 

 point. Mais ou Ton n'est pas d'accord, c'est sur la limite de sa 

 puissance et sur la question de savoir si elle peut engendrer des 

 formes specifiques nouvelles. 11 semble bien demontrer aujourd'hui 

 qu'elle ne le peut pas." 



4 "A study of the recent discussion in the Contemporary Reriezv 

 between Spencer and Weismann leads to the conclusion that neither 



