CAUSES OF VAEIATION 125 



in fission could not recur in each generation ; and if 

 Mr. Headley was right, no animal could have re- 

 mained so long unchanged as Lingula and other 

 well-known persistent types have done. Indeed the 

 most remarkable thing in fission is its extraordinary 

 regularity. If it was so irregular as Mr. Headley 

 supposes, or if it was an important cause of varia- 

 tion, organic nature would be out of joint in a few 

 generations. It may be a cause of malformations 

 or monstrosities, but hardly of ordinary variation. 



Physiogenesis. This may be due to the action 

 of the climate on the individual ; or to the chemical 

 action of food taken into the interior. Competition 

 with other organisms could not originate variations 

 by physiogenesis. 



The action of external causes on the organism is 

 sometimes easily recognised. For instance, the 

 growth of muscle by exercise, the thickening of the 

 skin by friction, the influence of continued pres- 

 sure on growth, and by climate preventing the 

 proper development of plants-. Other cases are much 

 more doubtful, such as the growth of spines on 

 plants in dry climates, the peculiar plumage of birds 

 inhabiting deserts, or the thickening of fur in cold 

 climates. We cannot explain why cold should 

 cause more hair follicles to be developed, or why it 

 should cause more rapid secretion of hair in the 

 follicles already in existence. Experience seems to 

 shew that such is the case, and the action is usually 

 said to be indirect. But that is a phrase which has 

 no distinct meaning. The association of two 

 things does not always prove that one is the cause 



