t
what seem to us identical conditions are really identical to such small
and delicate organisms as these land molluscs, of whose needs and
difficulties at each successive stage of their existence, from the
freshly-laid egg up to the adult animal, we are so profoundly ignorant.
The exact proportions of the various species of plants, the numbers of
each kind of insect or of bird, the peculiarities of more or less
exposure to sunshine or to wind at certain critical epochs, and other
slight differences which to us are absolutely immaterial and
unrecognisable, may be of the highest significance to these humble
creatures, and be quite sufficient to require some slight adjustments of
size, form, or colour, which natural selection will bring about. All we
know of the facts of variation leads us to believe that, without this
action of natural selection, there would be produced over the whole area
a series of inconstant varieties mingled together, not a distinct
segregation of forms each confined to its own limited area.

Mr. Darwin has shown that, in the distribution and modification of
species, the biological is of more importance than the physical
environment, the struggle with other organisms being often more severe
than that with the forces of nature. This is particularly evident in the
case of plants, many of which, when protected from competition, thrive
in a soil, climate, and atmosphere widely different from those of their
native habitat. Thus, many alpine plants only found near perpetual snow
thrive well in our gardens at the level of the sea; as do the tritomas
from the sultry plains of South Africa, the yuccas from the arid hills
of Texas and Mexico, and the fuchsias from the damp and dreary shores of
the Straits of Magellan. It has been well said that plants do not live
where they like, but where they can; and the same remark will apply to
the animal world. Horses and cattle run wild and thrive both in North
and South America; rabbits, once confined to the south of Europe, have
established themselves in our own country and in Australia; while the
domestic fowl, a native of tropical India, thrives well in every part of
the temperate zone.

If, then, we admit that when one portion of a species is separated from
the rest, there will necessarily be a slight difference in the average
characters of the two portions, it does not follow that this difference
has much if any effect upon the characteristics that are developed