he effect is 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, &c.: 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 {133} 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 generat