extremely small in the case of animals, but perhaps rather more in that
of plants. We may, at least, safely conclude that such influences
cannot have produced the many striking and complex co-adaptations of
structure between one organic being and another, which we see
everywhere throughout nature. Some little influence may be attributed
to climate, food, etc.: thus, E. Forbes speaks confidently that shells
at their southern limit, and when living in shallow water, are more
brightly coloured than those of the same species further north or from
greater depths. Gould believes that birds of the same species are more
brightly coloured under a clear atmosphere, than when living on islands
or near the coast. So with insects, Wollaston is convinced that
residence near the sea affects their colours. Moquin-Tandon gives a
list of plants which when growing near the sea-shore have their leaves
in some degree fleshy, though not elsewhere fleshy. Several other such
cases could be given.
The fact of varieties of one species, when they range
into the zone of habitation of other species, often acquiring in a very
slight degree some of the characters of such species, accords with our
view that species of all kinds are only well-marked and permanent
varieties. Thus the species of shells which are confined to tropical
and shallow seas are generally brighter-coloured than those confined to
cold and deeper seas. The birds which are confined to continents are,
according to Mr. Gould, brighter-coloured than those of islands. The
insect-species confined to sea-coasts, as every collector knows, are
often brassy or lurid. Plants which live exclusively on the sea-side
are very apt to have fleshy leaves. He who believes in the creation of
each species, will have to say that this shell, for instance, was
created with bright colours for a warm sea; but that this other shell
became bright-coloured by variation when it ranged into warmer or
shallower waters.
When a variation is of the slightest use to a being, we cannot tell how
much of it to attribute to the accumulative action of natural
selection, and how much to the conditions of life. Thus, it is well
known to furriers that animals of the same species have thicker and
better fur the more severe the climate is under which they have lived;
but who can tell how much of this difference may be due to the
warmest-clad individuals having been favoured and preserved during many
generations, and how much to the direct action of the severe climate?
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