Humming-Birds. - 217 
Hals, and lined inside with vegetable down. The 
eggs are white, and never exceed two in number. 
Broadly speaking, they resemble each other as 
closely in habits as in structure ; the greatest 
differences in habit in the most widely separated 
genera being no greater than may be found in two 
wrens or sparrows of the same genus. 
This persistence of character in humming-birds, 
both as regards structure and habit, seems the 
more remarkable when we consider their very wide 
distribution over a continent so varied in its con- 
ditions, and where they range from the lowest 
levels to the limit of perpetual snow on the Andes, 
and from the tropics to the wintry Magellanic 
district ; also that a majority of genera inhabit 
verv circumscribed areas — these facts, as Dr, 
Wallace remarks, clearly pointing to a very high 
antiquity. 
It is perhaps a law of nature that when a species 
(or group) fits itself to a place not previously 
occupied, and in which it is subject to no opposition 
from beings of its own class, or where it attains 
so great a perfection as to be able easily to over- 
come all opposition, the character eventually loses 
its original plasticity, or tendency to vary, since 
improvement in such a case would be superfluous, 
and becomes, so to speak, crystallized in that form 
which continues thereafter unaltered. It is, at any 
rate, clear that while all other birds rub together 
in the struggle for existence, the humming-bird, 
owing to its aerial life and peculiar manner of 
seeking its food, is absolutely untouched by this 
kind of warfare, and is accordingly as far removed 
