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.
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