330 DARWINISM TO-DAY. 



been recorded.) Species-forming by sports and discon- 

 tinuous variations is obviously no theory to presume to 

 offer itself as a species-forming substitute for natural selec- 

 tion. But the de Vriesian mutations theory, the most 

 recent development of the heterogenesis conception, has 

 rehabilitated this conception to such an extent that a number 

 of biologists see in it an actually satisfactory substitute for 

 the natural selection theory. Before explaining the theory 

 of de Vries let us first note two or three other prior formula- 

 tions of theories of heterogenesis, one at least being nearly 

 identical with that of de Vries. 



In 1864 the great zoologist von Kolliker, 9 in a paper under 



the title "t)ber die darwinische Schopfungstheorie," took 



positive ground against the adequacy or actuality 



VonE.olhker's Q na t ura i selection as a species- forming factor, 



suggestion, r D 



and proposed a theory of "heterogene Erzeu- 

 gung" (heterogenesis) which he formulated, however, only 

 in most general terms. He said that "under the influence 

 of a general law of development (evolution) organisms 

 bring forth other kinds differing from them out of the 

 germs produced by them." He included in his general theory 

 of heterogenesis a basic plan of progressive evolution. 

 Such a conception has in it too much autogenic orthogenesis ; 

 it is too redolent of teleology for present-day biology. The 

 variations, too, which are to serve as beginnings of new 

 species are those too rare ones which we have referred to 

 as sports and discontinuous variations. 



The American naturalist, Dall, T in a paper written in 

 1877, expresses his conviction that sudden changes of 



species-forming character do occur, and as- 



Dall's belief cribes such changes "to the action of the 



iies-chang^ 6 " ^ aw of development, which finds expression 



in the paradox that the same species may 

 belong to different genera." That sudden leaps may be 

 due to the gradual accumulation of minute differences he 



