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The proportions in which these four types appeared shows that the presence or absence of but two factors is concerned, and at the same time provides the key to the nature of the homostyled plants.

But how the sexual elements of heterostyled plants came to differ from what they were whilst the species was homostyled, and how they became co-adapted in two sets of individuals, are very obscure points.

Polygonum in possessing only a single heterostyled species is an extreme case; but every other genus of considerable size which includes some such species likewise contains homostyled species.

Mr. Scott has suggested that the equal-styled varieties arise through reversion to the former homostyled condition of the genus.

H. Muller has suggested that ordinary or homostyled plants may have been rendered heterostyled merely through the effects of habit.