vil ON THE INFERTILITY OF CROSSES 177 



prejudicial effect. If, for example, five per cent of each 

 form thus varied so as to be infertile with the other form, 

 the result would be hardly perceptible, because the individuals 

 which formed cross-unions and produced hybrids would con- 

 stitute a very small portion of the whole species ; and the 

 hybrid offspring, being at a disadvantage in the struggle for 

 existence and being themselves infertile, would soon die out, 

 while the much more numerous fertile portion of the two 

 forms would increase rapidly, and furnish a sufficient number 

 of pure-bred offspring of each form to take the place of the 

 somewhat inferior hybrids between them whenever the 

 struggle for existence became severe. "We must suppose that 

 the normal fertile forms would transmit their fertility to their 

 progeny, and the few infertile forms their infertility ; but 

 the latter would necessarily lose half their proper increase 

 by the sterility of their hybrid offspring whenever they 

 crossed with the other form, and when they bred with their 

 own form the tendency to sterility would die out except in 

 the very minute proportion of the five per cent (one-twentieth) 

 that chance would lead to pair together. Under these 

 circumstances the incipient sterility between the two forms 

 would rapidly be eliminated, and could never rise much above 

 the numbers which were produced by sporadic variation each 

 year. 



It was, probably, by a consideration of some such case as 

 this that Mr. Darwin came to the conclusion that infertility 

 arising between incipient species could not be increased by 

 natural selection ; and this is the more likely, as he was 

 always disposed to minimise both the frequency and the 

 amount even of structural variations. 



We have yet to notice another mode of action of natural 

 selection in favouring and perpetuating any infertility that 

 may arise between two incipient species. If several distinct 

 species are undergoing modification at the same time and in 

 the same area, to adapt them to some new conditions that 

 have arisen there, then any species in which the structural or 

 colour differences that have arisen between it and its varieties 

 or close allies were correlated with infertility of the crosses 

 between them, would have an advantage over the corre- 

 sponding varieties of other species in which there was no such 



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