104 THE CONTROL OF LIFE 



hard non-living parts of animals, such as the chitinous 

 tools of insects, are just as suggestive of direct adapta- 

 tions as the pads of camels, but the interpretation is 

 impossible, since they are not living or individually 

 plastic. 



The entailment of individually acquired modifications 

 seems such a ready method of securing progress in evolu- 

 tion, that if it does not occur one would like to know 

 why. The answer is not far to seek. If individually 

 acquired advantageous gains could be entailed, the same 

 would also apply to individual losses. Were transmis- 

 sion of modification on the credit side of the account, 

 would there not be the same on the debit side ? Had 

 there been any way of entailing modification gains, but 

 not modification losses, it would surely have been found 

 out long ago. There seems to be no way. Why are 

 modifications not transmitted ? Actually, because of 

 the absence of any arrangement, so far as we know, for 

 securing that modifications can affect the germ-cells in a 

 manner so specific that the offspring exhibit the same 

 modification, or some approximation towards it. It 

 has been suggested by Cunningham, Bergson, Dendy, 

 and MacBride that a modification may liberate a specific 

 hormone which affects the germ-cells representatively ; 

 and the same idea was in Darwin's mind in his theory 

 of gemmules liberated from the various parts of the 

 body and travelling to the germ-cells. But it must be 

 firmly pointed out that these are unverified speculative 

 interpretations of the possible accomplishment of what 

 we do not know to be a fact. From the point of view 



