Humming-Birds. 217 



rials, 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 

 very 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 



