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Proceedings of the Royal Society 
of flight, in the nature of the food, but runs also into some very 
curious details, as for example, in the number of feathers in the tail 
and in the wings, which are constant numbers — adhered to even 
when some of the feathers, not being used even for ornament, are 
reduced almost to rudiments. But under degrees of development 
which are very variable, the number is invariable. This identity of 
structure is the more remarkable from the immense extent of the 
group which it characterises. There are now knov/n to science no 
less than 41 6 different species of humming-bird ; and it cannot be 
doubted that many more remain to be discovered among the im- 
mense forests and mountain ranges of Central America. 
Now, what is the bond which unites so closely, in a common 
structure, all the forms of this great family of birds ? We think it a 
sufficient explanation sometimes of the likeness of things, that they 
are made for a common purpose. And so it is an explanation in one 
sense, but not in another. It gives the reason why likeness should 
be aimed at, but not the cause through which it has been brought 
about. Sameness in the purpose for which things are intended, is 
a reason why those things should be made alike ; but it is no ex- 
planation of the process to which the common aspect is due. It is 
an explanation of the “why but it is no explanation of the “how.” 
Purpose is attained in nature through the instrumentality of means ; 
and community of aspect in created things suggests the idea of 
some common process in the creative work. The likeness which 
is due to common parentage serves the most important purposes ; 
but it is not the less the result of a physical cause, out of which it 
arises by waj^ of natural consequence. The likeness of the Hum- 
ming-birds to each other suggests this kind of cause. It is true 
that the organs which it principally affects are specially adapted 
for a special habit of life. They are fitted to enable the bird to 
feed on the nectar, and the insects which frequent the nectar of 
flowers. But there are flowers in abundance in other quarters of 
the globe where there are no Humming-birds. And here we come 
on the curious facts of geographical distribution, — a class of facts 
which, as much as any other, suggest some specific methods as 
having been followed in the work of creation. Humming-birds 
are absolutely confined to the great continent of America with 
its adjacent islands. Within those limits there is every range 
