4 DEVELOPMENT AND HEREDITY. 



of evolution. According to them, in the severe 

 struggle for existence, every slightest variation must 

 tell ; no matter how slight, it must be either useful 

 or detrimental to the individual. Even the further 

 degeneration of an already useless and degenerate 

 organ is held to be advantageous as a saving of 

 energy, which will help the individual in the struggle 

 for existence. So acute is the action of natural 

 selection that each slight degeneration of the eye 

 of cave-dwelling animals is a useful variation — an 

 economy of energy — which is acted upon by nat- 

 ural selection. Also the degeneration of parasites 

 in the flesh of their hosts — where food is super- 

 abundant, and where warfare with fellow-parasites 

 does not occur — is also brought about by natural 

 selection for the purpose of saving energy, even 

 though energy in the shape of food be so abundant. 

 According to this view, while it is not denied that 

 environment may cause variation, it is not thought 

 that environment can in any way determine the 

 variation for the advantage of the individual, 

 which means that it is equally liable to be detri- 

 mental : and thus the whole matter of organic 

 evolution resolves itself into the one factor of 

 natural selection. Indeed, Professor Weismann ^ 

 has expressed the opinion that the keenest action 



1 Essays upon Heredity. Authorised trans., Oxford, 1889. 



