e" that they
chase butterflies "when other food is scarce." The quick eye of Mr. Wallace
failed to detect them in the act, as also to note any unusual abundance of
other insectivorous forms, which therefore, considering Mr. Wallace's zeal
and powers of observation, we may conclude do not exist. Moreover, even if
there ever has been an abundance of such, it is by no means certain that
they would have succeeded in producing the conformation in question, for
the effect of this peculiar curvature on flight is by no means clear. 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. de
la Soc. Imp. d'Acclimat_. tome viii. p. 351) stated "that young shells
taken from the shores of England and placed in the Mediterranean at once
altered their manner of growth, and formed prominent diverging rays _like
those on the shells of the proper Mediterranean oyster_;" also to Mr.
Meehan, as stating (_Proc. Acad. Nat. Sc. of Philadelphia_, Jan. 28, 1862)
"that twenty-nine kinds of American trees all differ from their nearest
European allies in _a similar manner_, leaves less toothed, buds and seeds
smaller, fewer branchlets," &c. These are striking examples indeed!

But cases of simultaneous and similar modifications abound on all sides.
Even as regards our own species there is a very genera