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Show 260 CONCLUDING REMARKS CHAP, VI. large number of seeds, and this obviously is a very con1mon case, heterosty led will have an ad vantage over direcious plants, as all the individuals of the fonner, whilst only half of the latter, that is the fen1ales, yield seeds. On the other hand, hetero~ tyled plants seem to have no advantage, as far as cross-fertilisation is concerned, over those which are sterile with their own pollen. They lie indeed under a slight disadvantage, for if two self-sterile plants grow near together and far removed from all other plants of the same species, they will mutually and perfectly fertilise one another, whilst this will not be the case with heterostyled dimorphic plants, unless they chance to belong to opposite forms. It may be added that species which are trimorphic have one slight advantage over the di1norphic; for if only two individuals of a dimorphic species happen to grow near together in an isolated spot, the chances are even that both will belong to the same form, and in this case 'they will not produce the full number of vigorous and fertile seedlings ; all these, moreover, will tend strong I y to belong to the same form as their parents. On the other hand, if two plants of the same trimorphic species happen to grow in an isolated spot, tho chances are two to one in favour of their not be· longing to the same form; and in ·this case they will legitimately fertilise one another, and yield the full complement of vigorous offspring. The Means by which Plants may have been rendered Heterostyled. This is a very obscure subject, on which I can throw little light, but which is worthy of discussion. It has CHAP. VI. ON HETEROS'rYLED PLANTS. 261 been shown that heterostyled plants occur in fourteen natural families, dispersed throughout the whole vegetable kingdom, and that even within the family of the Rubiacere they are dispersed in eight of the tribes. W o may therefore conclude that this structure has been acquired by various plants independently of inheritanco from a common progenitor, and that it can be acquired without any greit difficulty-that is, without any very unusual con1bination of circumstances. It is probable that the first step towards a species becoming heterostyled is great variability in the length of the pistil and stamens, or of the pistil alone. Such variations are not very rare: with Amsinckia spectabil£s and Nolana prostrata these organs differ so much in length in different individuals that, until experimenting on them, I thought both species heterostylerl. The stigma of Gesneria pendulina sometimes protrudeR far beyond, and is so1netimes seated beneath the anthers; so it is with Oxalis acetosella and varionR other plants. I have also noticed an extraordinary amount of difference in the length of the pistil in cultivated varieties of Primula veris and vulgaris. As most plants are at least occasionally cross-fertilised by the aid of insects, we 1nay assume that thiR was the case with our supposed varying plant; but that it would have been beneficial to it to have been more regularly cross-fertilised. We shoulJ. bear in mind how in1portant an advantage it has been proved to be to many plants, though in different degrees and ways, to be cross-fertilised. It might well happen that our supposed species did not vary in function in the right manner, so as to becoinA either dichoga1nous or completely self-sterile, or in structure so as to ensure cross-fertilisation. If it had |