Chap. XIX. HYBRIDISM. 169 



stitution. We are also led to this same conclusion by con- 

 sidering reciprocal crosses, in which the male of one species 

 cannot he united, or only with great difficulty, with the 

 female of a second species, whilst the converse cross can be 

 effected with perfect facility. That excellent observer, Gartner, 

 likewise concluded that species when crossed are sterile owing 

 to differences confined to their reproductive systems. 



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 spon- 

 taneously 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 advantageous, 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 (Mascari comosum) have been rendered conspicuous, 

 and apparently in consequence sterile, in order that insects 

 might easily discover and visit the perfect 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 



