Weismannism up to date (1893). 147 



Weismanris Theory of Evolution (1893). 



Of far more importance than any of the alterations 

 which Professor Weismann has recently made in his 

 theory of heredity, are those whereby he has modi- 

 fied his sequent theory of evolution. For while, 

 as we have just seen, his work on The Germ-plasm 

 leaves the former theory substantially unaltered, — 

 although largely added to in matters of detail. — it 

 so profoundly modifies the latter that careful readers 

 will find no small difficulty in ascertaining how much 

 of it has been allowed to remain. I will consider 

 only the main modifications, and these I will take 

 separately. 



It will be remembered that one distinctive feature 

 in Weismann's theory of evolution has hitherto been, 

 that the unicellular organisms differ from the multi- 

 cellular in the following important particulars. 



i. There being no division in unicellular organisms 

 between germ-cells and somatic-cells, there is no 

 possibility in them of the occurrence of amphimixis. 



2. Consequently, there is no possibility in them of 

 congenital variations, in the sense that these occur 

 in multicellular organisms. 



3. Hence the only causes of individual variation 

 and of the origin of species in the unicellular organ- 

 isms are the Lamarckian factors, just as in the multi- 

 cellular the only cause of these things is natural 

 selection. 



4. Hence, also, the unicellular organisms are paten- 



ts 2 



