Attributes of Successful Species 
to which they are unaccustomed will become 
emaciated and perish. We see this in every hard 
winter in England, when the redwing, which, 
unlike other thrushes, cannot thrive on berries, is 
the first to die. Most of the more successful 
birds—the crows and gulls, for example—are 
omnivorous—that is to say, they are able to 
digest all manner of food. 
Under mental capacity, we would include 
cunning and sufficient intelligence to adapt one- 
self to changed conditions. It is largely through 
man’s superior mental capacity that he has 
~ become the dominant species. It is true that 
he displays also courage and a good constitution, 
being able to adapt himself to life under the most 
diverse conditions ; but this is, of course, in part 
due to his mental capacity, which enables him 
to some extent to adapt his environment to 
himself. 
The advantages of prolificacy are so apparent 
that it is unnecessary to dilate upon them. 
Nearly as important as excessive fertility is the 
ability on the part of the parents to look after their 
young ones. si 
Every successful species possesses in a special 
degree at least one of the above attributes. It 
is interesting to take in turn the various species 
which are most widely distributed and consider 
to what extent they possess these several 
qualities. 
355 
