Chap. XIX. HYBRIDISM. 185 



species depends exclusively on differences in their sexual con- 

 stitution. 



On the principle which makes it necessary for man, whilst he 

 is selecting and improving his domestic varieties, to keep them 

 separate, it would clearly be advantageous to varieties in a state 

 of nature, that is to incipient species, if they could be kept 

 from blending, either through sexual aversion, or by becoming 

 mutually sterile. Hence it at one time appeared to me probable, 

 as it has to others, that this sterility might have been acquired 

 through natural selection. On this view we must suppose that 

 a shade of lessened fertility first spontaneously appeared, like 

 any other modification, in certain individuals of a species 

 when crossed with other individuals of the same species ; and 

 that successive slight degrees of infertility, from being advan- 

 tageous, were slowly accumulated. This appears all the more 

 probable, if we admit that the structural differences between 

 the forms of dimorphic and trimorphic plants, as the length 

 and curvature of the pistil, &c, have been co-adapted through 

 natural selection ; for if this be admitted, we can hardly avoid 

 extending the same conclusion to their mutual infertility. 

 Sterility moreover has been acquired through natural selection 

 for other and widely different purposes, as with neuter insects 

 in reference to their social economy. In the case of plants, the 

 flowers on the circumference of the truss in the guelder-rose 

 ( Viburnum opulus) and those on the summit of the spike in the 

 feather-hyacinth (Muscari comosum) have been rendered con- 

 spicuous, and apparently in consequence sterile, in order that 

 insects might easily discover and visit the other flowers. But 

 when we endeavour to apply the principle of natural selection 

 to the acquirement by distinct species of mutual sterility, we 

 meet with great difficulties. In the first place, it may be 

 remarked that separate regions are often inhabited by groups of 

 species or by single species, which when brought together and 

 crossed are found to be more or less sterile ; now it could clearly 

 have been of no advantage to such separated species to have 

 been rendered mutually sterile, and consequently this could not 

 have been effected through natural selection ; but it may per- 

 haps be argued, that, if a species were rendered sterile with 



