CHAPTER VI 

 RETROGRESSION 



Evidence of retrogression Theories of retrogression Conditions under which 

 retrogression occurs The speed of retrogression The difficulty of recognizing 

 the identity of retrogression and reversion Latent characters The retrogression 

 of variations The magnitude of the part played by retrogression Retrogres- 

 sion in the developing individual Reversed selection ' Ancestral units ' The 

 ' contributions of ancestors ' The law of ancestral inheritance Biometry. 



X 73- ^ I CHOUGH, speaking practically, the doctrine of evolu- 

 tion is accepted by all who know the facts, that of 

 "^ evolution by Natural Selection has not as yet received 

 such general assent. A vague terminology, and a belief that 

 nutritional characters are peculiarly ' innate ' and that ' acquire- 

 ments ' are not products of evolution, combined with the obvious- 

 ness of the Lamarckian doctrine and the difficulty of explaining 

 co-adaptation when the magnitude of * use-acquirements ' is not 

 realized, still inclines some biologists to a faith that the ' inheritance ' 

 of acquirements accounts, in part at least, for evolution. Others, 

 because they are unable to perceive the utility of some charac- 

 ters in types so far removed from the human that our know- 

 ledge of their dangers and activities is very imperfect, have 

 supposed that such characters have no utility, that they are not 

 adaptations, and, therefore, that they do not owe their evolution to 

 Natural Selection, which, consequently, does not play an exclusive, 

 but, on the contrary, only a more or less minor part in the causa- 

 tion of specific change. 1 Yet others, because a few species, when 

 the environment is changed, display quite suddenly considerable 

 adaptive alterations, pin their faith to that circumlocution for 

 miracle, an ' adaptive growth-force.' 2 



174. But perhaps the main reason against a full acceptance of 

 the Darwinian doctrine that, at any rate, which has influenced 

 most biologists in the past has been drawn from the retrogression 

 of useless parts and organs. In all complex types occur traces of 

 organs which formerly were useful and of considerable relative 



1 See 306, 649. 2 See 92. 



107 



