hearing on Geology a/nd Zoogeography. 69 



whole," is utterly unwarranted. J^atural selection can never 

 explain the divergence of the directions of mutation, that is to 

 say, the origin of separate forms from one ancestral form: 

 this is only conceivable if we introduce the principle of sepa- 

 ration. But further it will be remarked, that I lay much stress 

 upon the transforming power of the external conditions of life, 

 and that I differ also in this respect from the theories pro- 

 pounded by Weismann, who does not believe, that external 

 conditions are able to affect the body of organic forms so that 

 the changes thus produced may be transmitted to descendants 

 and give origin to a change of the specific characters. In this 

 respect I may mention here, that I am a decided opponent to 

 Weismann's theories as a whole, and that I consider them to 

 be illogical and contrary to the known facts, which is — in my 

 opinion — amply demonstrated already by Pfeffer and O. Hert- 

 wig. I refer only to these critics among the large number of 

 Weismann's adversaries, especially since Pfeffer^ has engaged 

 himself in demonstrating the logical contradictions of Weis- 

 mann, and O. Hertwigf in demonstrating the contradictions of 

 Weismann's theories to the facts. Perhaps, I shall find oppor- 

 tunity to contribute to the criticism of Weismann's theories 

 elsewhere : it may be sufficient here to say, that I cannot accept 

 Weismann's theories of the origin of variations, and accord- 

 ingly, that I must stick up for the old " working hypothesis," 

 that variations are produced by the surrounding conditions, and 

 that these variations may be transmitted. 



Princeton, N. J., March, 1896. 



*Die inneren Fehler der Weismannschen Keimplasma-Theorie. — Yerhandl. 

 Naturwiss. Ver. Hamburg (3) I. 1894. 



f Zeit- und Streitfragen der Biologie., Heft 1, Praformation oder Epigenese, 

 Jena, 1894. 



