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? for it would appear that climate has some direct action on
the hair of our domestic quadrupeds.
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