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. 
