110 THE CELL IN RELATION TO 



" entelechy " or " elan de la vie," a power of un- 

 known nature, that may, in the last analysis, be 

 psychical in nature. But, profoundly interest- 

 ing as some of these vitaKstic speculations are, we 

 are bound to hold fast to the physico-chemical and 

 mechanistic hypothesis of heredity until the pos- 

 sibilities of observation and experiment in this 

 direction have been exhausted. If there be a 

 physico-chemical basis of heredity we should ex- 

 pect to find it capable of modification by physico- 

 chemical agencies ; and so much, at least, is known 

 to be the fact. It has been abundantly demon- 

 strated that both the body-cells and the germ- 

 cells react to changes of the environment by def- 

 inite physiological and morphological changes. 

 Many experimenters have demonstrated the ex- 

 treme susceptibility of the discharged eggs or 

 spermatozoa to even very shght chemical and 

 physical stimuh. We can not doubt that they are 

 equally sensitive to stimuli while still within the 

 body, and at every stage of their development. 

 The almost unique experiments of MacDougal 

 on the higher plants seem to show that direct 

 chemical treatment of the germ-cells may pro- 

 duce definite and irreversible effects upon the off- 

 spring. Those of Tower on the insects, though 

 less direct, are hardly less convincing. 



Though we may not fully understand the man- 

 ner in which the germ-cells are modified, there is 

 no inherent improbability or difficulty in the con- 

 ception that such modifications will produce bias- 



