We have here, then, a structure hypothetically explained by an uncertain {88} property induced by a cause the presence of which is only conjectural.
Surely it is not unreasonable to class this instance with the others before given, in which a common modification of form or colour coexists with a certain geographical distribution quite independently of the destructive agencies of animals.

If physical causes connected with locality can abbreviate or annihilate the tails of certain butterflies, why may not similar causes produce an elbow-like prominence on the wings of other butterflies?
There are many such instances of simultaneous modification.
Mr.Darwin himself[67] quotes Mr.Gould as believing that birds of the same species are more brightly coloured under a clear atmosphere, than when living on islands or near the coast.

Mr.Darwin also informs us that Wollaston is convinced that residence near the sea affects the colour of insects; and finally, that Moquin-Tandon gives a list of plants which, when growing near the sea-shore, have their leaves in some degree fleshy, though not so elsewhere.

In his work on "Animals and Plants under Domestication,"[68] Mr.Darwin refers to M.Costa as having (in _Bull.

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