ig2i.] Si(hsj)ecies and Kvolution. 533 



can be ascribed to niututioii. I believe they are all due to 

 environment or isolation, both being geographical factors. 

 A mutation has nothing whatever to do with geography. 



I have tried to apply the mutation theory to species which 

 contain undoubted geographical races. I£ I were to ask you 

 to believe that mutation is responsible for geographical 

 variation, I should have to ask you to believe that British 

 races of Continental species all became smaller and darker 

 quite suddenly and quite accidentally; I should have to ask 

 you to accept that Egyptian Delta races all became dark 

 by accident and spontaneously. I should have to ask you 

 to believe that intermediate races in intermediate areas 

 are accidental. You would have to swallow the fact that 

 most pure desert species {Akemon, Ammomanes, Pterocles, 

 Eremopldla, (Enanthe deserti, Cxirsorius, and many others) 

 are of the same sandy hue on their upper parts by accident, 

 and are all descendants of " sports." 



Take the Song-Thrush. The British race is darker than 

 the Continental race. The Hebridean race is even darker 

 than the British race. The degree o£ colour is in close 

 relation to the degree of rainfall in the breeding-quarters of 

 the three races. Is that accident ? If so, similar remark- 

 able accidents have occurred under similar conditions in 

 many parts of the world, which would be preposterous to 

 describe as a coincidence. 



I shall not even attempt to convince you of such fallacies, 

 as the theme is ridiculous. Evolution does not consist in 

 the perpetuation of a series of defective freaks. 



The advocate of mutation may say that such variations 

 have proved of value to the bird, and have been perpetuated 

 whilst other less suitable variations have been rejected by 

 natural selection. But if that w^ere the case, why does the 

 Continental Song-Thrush spend from September to April in 

 the British Isles, if the climate is so unsuited to its 

 characters? And why do a host of other birds from the 

 north spend the autumn, winter^ and spring among their 

 more southern representatives, if the area of the latter is so 

 unsuited to the characters of the former ? 



