128 THE LESSON OF EVOLUTION 



acaulis is especially conservative, and still often 

 flowers in New Zealand in May at the same time 

 that its relations are flowering in Europe, although 

 it also flowers in the spring, that is in October. 



Physiogenesis is no doubt a cause of variation ; 

 but the theory supplies no method of action, and a 

 great many variations are included under this 

 heading which do not really belong to it, for they 

 are only indirectly due to changes, in external con- 

 ditions. Usually we cannot shew any connection 

 between cause and effect, and this leaves a doubt? 

 in our mind. And if the external causes are as 

 efficient as some suppose, surely we ought to be able 

 to trace their effects without such difficulty. Prof. 

 H. W. Conn says " We do not understand how en- 

 vironment can act upon the individual in such a, 

 way as to produce even acquired adaptive changes 

 in it. Why a muscle grows with use or diminishes 

 with disuse, why sensations become more acute 

 when exercised, why changes in food or climate 

 modify colours, why the shapes of leaves and the 

 length in the beaks of birds cnange with climate,, 

 we have not the faintest notion." 31 



If it is difficult to trace the action of the environ- 

 ment on the organism at the present day, when we 

 can observe their habits, how much more difficult 

 must it be to estimate its action on extinct animals. 

 And yet there are naturalists bold enough to say 

 that the changes in Ammonites were due to the 

 favourable nature of the physical surrounding in the 

 Jurassic period. 



31 " The Method of Evolution," 1900, p. 303. 



