70 THE EVOLUTIONARY THEORY 



writers, but his elaborate experiments and careful 

 arguments have for the first time given it a recognized 

 place among scientific theories — one that threatens 

 to destroy the influence of neo-Darwinism. You will 

 remember that Darwin and the neo-Dar^vinians make 

 slight, fluctuating, and continuous variations the basis 

 of the process of natural selection, and maintain that 

 a new species is formed by means of such selection, and 

 by a gradual accentuation and accumulation of the 

 variations selected. The new theory excludes natural 

 selection from any part in species-forming. Accord- 

 ing to it a new species is produced by sudden and dis- 

 continuous variations. These variations may be slight 

 in degree when severally considered, but a sufficient 

 number of them coincide to produce immediately by 

 their cumulative effect an independent species, suffi- 

 ciently differentiated to escape the swamping effects 

 of inter-breeding, and capable of perpetuating itself. 

 Biologic isolation is complete, and the number of 

 members of the new species which are thus produced 

 is also sufficient for their self-propagation. 



The new theory is thus directly opposed to Darwin- 

 ism in its explanation of the formation of new species. 

 But it leaves a place for natural selection in determin- 

 ing the general course of natural evolution. DeVries 

 says: *' 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 organ- 

 isms of minor worth through the struggle for life. It 

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



