ting-point and cause of the development
of new species, we have already found a reason why it should so often
appear when species become fully differentiated.

In almost all the cases of infertility or sterility between varieties or
species, we have some external differences with which it is correlated;
and though these differences are sometimes slight, and the amount of the
infertility is not always, or even usually, proportionate to the
external difference between the two forms crossed, we must believe that
there is some connection between the two classes of facts. This is
especially the case as regards colour; and Mr. Darwin has collected a
body of facts which go far to prove that colour, instead of being an
altogether trifling and unimportant character, as was supposed by the
older naturalists, is really one of great significance, since it is
undoubtedly often correlated with important constitutional differences.
Now colour is one of the characters that most usually distinguishes
closely allied species; and when we hear that the most closely allied
species of plants are infertile together, while those more remote are
fertile, the meaning usually is that the former differ chiefly in the
_colour_ of their flowers, while the latter differ in the form of the
flowers or foliage, in habit, or in other structural characters.

It is therefore a most curious and suggestive fact, that in all the
recorded cases, in which a decided infertility occurs between varieties
of the same species, those varieties are distinguished by a difference
of colour. The infertile varieties of Verbascum were white and yellow
flowered respectively; the infertile varieties of maize were red and
yellow seeded; while the infertile pimpernels were the red and the blue
flowered varieties. So, the differently coloured varieties of
hollyhocks, though grown close together, each reproduce their own colour
from seed, showing that they are not capable of freely intercrossing.
Yet Mr. Darwin assures us that the agency of bees is necessary to carry
the pollen from one plant to another, because in each flower the pollen
is shed before the stigma is ready to receive it. We have here,
therefore, either almost complete sterility between varieties of
different colours, or a prepotent effect of pollen from a flower of the
same colour, bringing about the same result.

Similar phenomena have not been recorded among animals; but this is not
to be wondered at when we