ORGANIC REGULATION 111 



arranged, or separated entirely from one another, a 

 complete embryo may still develop, even from a single 

 cell. He argues from this and other facts of analogous 

 character, (1) that any mechanistic explanation of life 

 is unthinkable, and (2) that we must assume the inter- 

 ference of a guiding influence, "entelechy," which 

 directs the material present, so that it develops in the 

 right way. 



Driesch's destructive criticism of the mechanistic 

 theory is particularly searching and cogent, and it 

 seems to me that both he and the older vitalists have 

 been justified up to the hilt in refusing to accept this 

 theory. In the previous part of this lecture I have 

 endeavoured to express the vitalistic criticism in a 

 still more general form than it has assumed in the 

 writings of the vitalists. To me the mechanistic theory 

 of life appears impossible, not merely in connection 

 with the facts of heredity and embryology, but at 

 every point in biology. 



To the vitalistic theory itself, however, there are 

 insuperable objections. Experience shows us that 

 where an organism reacts in any way it is always in 

 response to some stimulus, whether this stimulus origi- 

 nates from without or within. The stimulus of fertili- 

 sation normally initiates the segmentation of an ovum, 

 and from all analogy we must conclude that the differ- 

 ential stimuli arising from neighbouring cells or other 

 parts determine the subsequent differential behaviour 

 of each cell in the segmented ovum. On separating the 

 cells these differential stimuli are removed, and each 

 cell naturally starts again from the beginning. 



