158 DARWINISM chap. 



But an insect visiting, first, a long-styled plant, Avould deposit 

 the pollen on the stigma of another plant of the same kind if 

 it were next visited ; and this is probably the reason why the 

 wild short-styled plants were found to be almost always most 

 productive of seed, since they must be all fertilised by the 

 other form, whereas the long- styled plants might often be 

 fertilised by their own form. The whole arrangement, 

 however, ensures cross-fertilisation ; and this, as Mr. Darwin 

 has shown by copious experiments, adds both to the vigour 

 and fertility of almost all plants as well as animals. 



Besides the primrose family, many other plants of several 

 distinct natural orders present similar phenomena, one or 

 two of the most curious of which must be referred to. The 

 lieautiful crimson flax (Linum grandiflorum) has also two 

 forms, the styles only differing in length ; and in this case Mr. 

 Darwin found by numerous experiments, Avhich have since 

 been repeated and confirmed by other observers, that each 

 form is absolutely sterile Avith pollen from another plant of 

 its own form, but abundantly fertile when crossed with any 

 plant of the other form. In this case the pollen of the two 

 forms cannot be distinguished under the microscope (whereas 

 that of the two forms of Primula diff"ers in size and shape), 

 yet it has the remarkable property of being absolutely 

 powerless on the stigmas of half the plants of its own species. 

 The crosses between the opposite forms, which are fertile, are 

 termed by Mr. Darwin "legitimate," and those between 

 similar forms, which are sterile, "illegitimate"; and he 

 remarks that we have here, within the limits of the same 

 species, a degree of sterility which rarely occurs except 

 between plants or animals not only of different species but of 

 diff"erent genera. 



But there is another set of plants, the trimorphic, in which 

 the styles and stamens have each three forms — long, medium, 

 and short, and in these it is possible to have eighteen diff"erent 

 crosses. By an elaborate series of experiments it was shown 

 that the six legitimate unions — that is, when a plant was 

 fertilised by pollen from stamens of length corresponding to 

 that of its style in the two other forms — were all abundantly 

 fertile ; while the twelve illegitimate iinions, Avhen a plant was 

 fertilised by pollen from stamens of a different length from its 



