533 
1921 .] Subspecies and Evolution. 
can be ascribed to mutation. 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. If 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 ( Alcemon , Ammomanes, P ter odes, 
Eremophila , CEnanthe deserti, Cursorius , 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 of 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 were 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 ? 
