380 
TROPICAL NATURE 
IV 
development, the forms and colours, of these wonderful little 
birds — if we consider all these varied and complex influences, 
we shall be less surprised at their strange forms, their infinite 
variety, their wondrous beauty. For how many ages the 
causes above enumerated may have acted upon them we 
cannot say ; but their extreme isolation from all other birds, 
no less than the abundance and variety of their generic and 
specific forms, clearly point to a very high antiquity. 
The Relations and Affinities of Humming-birds 
The question of the position of this family in the class 
of birds and its affinities or resemblances to other groups 
is so interesting, and affords such good opportunities for 
explaining some of the best established principles of classifica- 
tion in natural history in a popular way, that we propose to 
discuss it at some length, but without entering into technical 
details. 
There is in the Eastern hemisphere, especially in tropical 
Africa and Asia, a family of small birds called sun-birds, which 
are adorned with brilliant metallic colours, and which, in shape 
and general appearance, much resemble humming-birds. They 
frequent flowers in the same way, feeding on honey and insects ; 
and all the older naturalists placed the two families side by 
side as undoubtedly allied. In the year 1850, in a general 
catalogue of birds, Prince Lucien Bonaparte, a learned 
ornithologist, placed the humming-birds next to the swifts, 
and far removed from the Nectarinidse or sun-birds ; and this 
view of their position has gained ground with increasing 
knowledge, so that now all the more advanced ornithologists 
have adopted it. Before proceeding to point out the reasons 
for this change of view, it will be well to discuss a few of the 
general principles which guide naturalists in the solution of 
such problems. 
How to Determine doubtful Affinities 
It is now generally admitted that, for the purpose of 
determining obscure and doubtful affinities, we must examine 
by preference those parts of an animal which have little or no 
direct influence on its habits and general economy. The value 
of an organ, or of any detail of structure, for purposes of 
