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 consider that most of our pure and valued domestic breeds are characterised by definite colours which constitute one of their
