The analogous facts in plants also indicate a real constitutional relation with colour, not an affection of the sense of smell and taste only.] [Footnote 60: For all these facts, see _Animals and Plants under Domestication_, vol.ii.pp.

335-338.] [Footnote 61: _Animals and Plants under Domestication_, vol.ii.

pp.
102, 103.] [Footnote 62: As this argument is a rather difficult one to follow, while its theoretical importance is very great, I add here the following briefer exposition of it, in a series of propositions; being, with a few verbal alterations, a copy of what I wrote on the subject about twenty years back.

Some readers may find this easier to follow than the fuller discussion in the text:-- _Can Sterility of Hybrids have been Produced by Natural Selection ?_ 1.

Let there be a species which has varied into _two forms_ each adapted to certain existing conditions better than the parent form, which they soon supplant.
2.