210 Darwin, and after Darwin. 



changes of size and colour among birds and mammals 

 on the American continent ; and he finds a won- 

 derfully close sliding scale of both, corresponding 

 stage by stage with gradual changes of climate. 

 Very reasonably he attributes this to the direct 

 influence of climatic conditions, without reference 

 to natural selection as does also Mr. Gould with 

 reference to similar facts which he has observed 

 among the birds of Australia. Against this view 

 Mr. Wallace urges, "that the effects are due to the 

 greater or less need of protection." But it is difficult 

 to believe that such can be the case where so in- 

 numerable a multitude of widely different species 

 are concerned presenting so many diverse habits, 

 as well as so many distinct habitats. Moreover, the 

 explanation seems incompatible with the graduated 

 nature of the change, and also with the fact that not 

 only colouration but size, is implicated. 



We meet with analogous facts in butterflies. 

 Thus Lycaena agestis not only presents seasonal 

 variations, (A) and (B) ; but while (A) and (B) are 

 respectively the winter and summer forms in 

 Germany, (B) and (C) are the corresponding forms 

 in Italy. Therefore, (B) is in Germany the summer 

 form, and in Italy the winter form the German 

 winter form (A) being absent in Italy, while the 

 Italian summer form (C) is absent in Germany. 

 Probably these facts are due to differences of tem- 

 perature in the two countries, for experiments have 

 shown that when pupae of sundry species of moths 

 and butterflies are exposed to different degrees of 

 temperature, the most wonderful changes of colour 

 may result in the insects which emerge. The re- 



