at the Conditions of Life
act in a potent manner in causing definite Modifications of Structure._

I have alluded to the slight differences in species when naturally living
in distinct countries under different conditions; and such differences we
feel at first inclined, probably to a limited extent with justice, to
attribute to the definite action of the surrounding conditions. But it must
be borne in mind that there are a far greater number of animals and plants
which range widely and have been exposed to great diversities of
conditions, yet remain nearly uniform in character. Some authors, as
previously remarked, account for the varieties of our culinary and
agricultural plants by the definite action of the conditions to which they
have been exposed in the different parts of Great Britain; but there are
about 200 plants[710] which are found in every single English county; these
plants must have been exposed for an immense period to considerable
differences of climate and soil, yet do not differ. So, again, some birds,
insects, other animals, and plants range over large portions of the world,
yet retain the same character.

Notwithstanding the facts previously given on the occurrence of highly
peculiar local diseases and on the strange modifications of structure
in plants caused by the inoculated poison of insects, and other
analogous cases; still there are a multitude of variations--such as the
modified skull of the niata ox and bulldog, the long horns of Caffre
cattle, the conjoined toes of the solid-hoofed swine, the immense crest
and protuberant skull of Polish fowls, the crop of the pouter-pigeon,
and a host of other such cases--which we can hardly attribute to the
definite action, in the sense before specified, of the external
conditions of life. No doubt in every case there must have been some
exciting cause; but as we see innumerable individuals exposed to nearly
the same conditions, and one alone is affected, we may conclude that
the constitution of the individual is of far higher {286} importance
than the conditions to which it has been exposed. It seems, indeed, to
be a general rule that conspicuous variations occur rarely, and in one
individual alone out of many thousands, though all may have been
exposed, as far as we can judge, to nearly the same conditions. As the
most strongly marked variations graduate insensibly into the most