d than those of the same species from further north or from a
greater depth; but this certainly does not always hold good. Mr. Gould
believes that birds of the same species are more brightly coloured under
a clear atmosphere, than when living near the coast or on islands; and
Wollaston is convinced that residence near the sea affects the colours
of insects. 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. These slightly varying organisms are interesting in
as far as they present characters analogous to those possessed by the
species which are confined to similar conditions.

When a variation is of the slightest use to any being, we cannot tell
how much to attribute to the accumulative action of natural selection,
and how much to the definite action of the conditions of life. Thus, it
is well known to furriers that animals of the same species have thicker
and better fur the further north they live; 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
action of the severe climate? For it would appear that climate has some
direct action on the hair of our domestic quadrupeds.

Instances could be given of similar varieties being produced from the
same species under external conditions of life as different as can well
be conceived; and, on the other hand, of dissimilar varieties being
produced under apparently the same external conditions. Again,
innumerable instances are known to every naturalist, of species keeping
true, or not varying at all, although living under the most opposite
climates. Such considerations as these incline me to lay less weight on
the direct action of the surrounding conditions, than on a tendency to
vary, due to causes of which we are quite ignorant.

In one sense the conditions of life may be said, not only to cause
variability, either directly or indirectly, but likewise to include
natural selection, for the conditions determine whether this or that
variety shall survive. But when man is the selecting agent, we clearly
see that the two elements of change are distinct; variability is in
some manner excited, but it is the will of man which accumulates the
variations in certain direction; and it is this latter agency which
answers to the survival of the fittest under nature.

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