No. 579] PROGRESSIVE EVOLUTION 



the bottom of things in any department of knowledge; 

 there is always something remaining beyond our reach. 

 If we are asked why an organism chooses the most ap- 

 propriate response to any particular stimulus, we may 

 suggest that this is the response that relieves it from 

 further stimulation, but we cannot say how it learns to 

 choose that response at once in preference to all others. 

 If we are asked to account for some particular muta- 

 tion, we may say that it is due to some modification in 

 the constitution or distribution of the chromosomes in 

 the germ-cells, but even if we knew exactly what that 

 modification was, and could express it in chemical terms, 

 we could not really say why it produces its particular 

 result and no other, any more than the chemist can say 

 why the combination of two gases that he calls oxygen 

 and hydrogen gives rise to a liquid that he calls water. 



There is one group of ontogenetic phenomena in par- 

 ticular that seems to defy all attempts at mechanistic 

 interpretation. I refer to the phenomena of restitution, 

 the power which an organism possesses of restoring 

 the normal condition of the body after it has been vio- 

 lently disturbed by some external agent. The fact that 

 a newt is able to regenerate its limbs over and over 

 again after they have been removed, or that an echino- 

 derm blastula may be cut in half and each half give rise 

 to a perfect larva, is one~ of the most surprising things 

 in the domain of biological science. We can not, at 

 present, at any rate, give any satisfactory mechanistic 

 explanation of these facts, and to attribute them to the 

 action of some hypothetical entelechy, after the manner 

 of Professor Hans Driesch, is simply an admission of 

 our inability to do so. We can only say that in the 

 course of its evolution each organism acquires an indi- 

 viduality or wholeness of its own, and that one of the 

 fundamental properties of living organisms is to main- 

 tain that individuality. They are able to do this in a 

 variety of ways, and can sometimes even replace a lost 



