279 
of Edinburgh^ Session 1864-65. 
of climate, and there are particular species of Humming-bird 
adapted to every region where a flowering vegetation can sub- 
sist. It is therefore neither climate nor food which confines the 
Humming-birds to the New World. What is it, then? The idea 
of “centres of creation” is at once suggested to the 'mind. It 
seems as if the Humming-birds were introduced at one spot, and 
as if they had spread over the whole continent which was ac- 
cessible to them from that spot. They are absent elsewhere, 
simply because from that spot the other continents of the world 
were inaccessible to them. But if these ideas are suggested to the 
mind by the general aspect of this family as a whole, they are 
strengthened by some of the facts wdiich we discover when we 
examine and compare with each other the genera and species of 
which it is composed. There is a beautiful gradation between the 
different genera and the different species, so much so, that it 
has been found impossible to divide the Humming-birds into 
more than two sub-families, from the absence of sufficiently well- 
marked divisions. ■^.And yet, on the other hand, they cannot be 
arranged in anything like a continuous series, because some 
links appear to be missing in the chain. 
But these general facts terminate in nothing more definite than 
a vague surmise. When we enter farther into details, we feel at 
once how little they agree with any physical law which is known or 
even conceivable by us. If the likeness which prevails in the 
whole group reminds us of the likeness which is due to community 
of blood, it is equally true that the differences between the species 
are totally distinct both in kind and degree from the variation 
which we ever see arising amoug the offspriug of the same parents. 
Let us look at what these differences are. The generic and 
specific distinctions between the humming-birds are mainly of 
two kinds, — Ist^ Differences in the form of essential organs, such 
as the bill and the wings ; 2c?, Differences in those parts of the 
plumage which are purely ornamental. Now, of these two kinds of 
variation, the only one on which the law of natural selection has 
any bearing at all, is the first. And on that kind of variation, the 
only bearing which natural selection has is this — that if any 
Humming-bird were born with a new form of bill, or a new form of 
wing, which enabled it to feed better and to range farther, that 
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VOL. V. 
