HEREDITY AND EVOLUTION 111 



togenic variations or mutations that are inherited, 

 permanently or temporarily. We can readily 

 understand that the constitutional effects of tem- 

 perature, food, moisture, and similar general? 

 agencies of the environment may manifest them- 

 selves in definite changes that reappear in follow- i 

 ing generations because the germ-cells have been\ 

 directly affected in the same way as the somatic 

 cells. It is natural to suppose that the idioplasm | 

 possesses a slight instability of chemical or molec- i 

 ular composition that results in corresponding 

 fluctuations or indefinite variations of the adult,! 

 which may or may not be inherited. We find no| 

 difficulty in the conception that the idioplasm 

 may xmdergo considerable, sudden, and irre- 

 versible changes which produce mutations of 

 greater or less degree. We can comprehend how 

 particular constituents of the idioplasm may 

 change without affecting others, thus giving rise 

 to mutations in respect to only a single character 

 or a particular group of characters. We can con- 

 ceive the idioplasm as undergoing a slow secular 

 change that results in continual divergence in 

 many directions or in a definite orthogenetic line 

 of transformation. But in respect to the trans- i 

 mission of acquired characters the old difficulty 

 confronts us to-day as formidable as when it was 

 first fairly revealed to us through the argument 

 of Weismann. What is really difficult to com- 

 prehend, what I think we can not really conceive 

 if pangenesis be discarded, is how the idioplasm. 



