THE CROSS-BREEDING OF RACES 145 



fertility, which is transmitted in an exceptional degree to 

 the races or varieties which are experimentally inter-bred, 

 and, consequently, may be expected to produce fertile 

 mongrels. Alfred Russel Wallace insisted upon this fact, 

 and pointed out that in a few cases colour varieties of a 

 given species of plant have been found to be incapable of 

 inter-breeding, or only produce very few " mongrels." 

 This has been established in the case of two dissimilarly- 

 coloured varieties of mullein. Also the red and the blue 

 pimpernel (the poor man's weather-glass, Anagallis), which 

 are classed by botanists as two varieties of one species, 

 have been found after repeated trials to be definitely in- 

 capable of inter-breeding. Wallace insists in regard to 

 crossing, that some degree of difference favours fertility, 

 but a little more tends to infertility. We must remember 

 that the fertility of both plants and animals is very easily 

 upset. Changed conditions of life — such as domestication 

 — may lead (we do not know why) to complete or 

 nearly complete infertility; and, again, " change of air," 

 or of locality, has an extraordinary and not-as-yet- 

 explained effect on fertility. 



" Oh, the little more and how much it is ! 

 And the little less, and what worlds away ! " 



Infertile horses sent from their native home to a different 

 climate (as, for instance, from Scotland to Newmarket) 

 become fertile. A judicious crossing of varieties or races 

 threatened with infertility will often lead to increased vigour 

 and fertility in the new generation, just as change of locality 

 will produce such a result. Physiological processes which 

 are not obvious and cannot be exactly estimated or 

 measured are then, we must conclude, largely connected 

 with the question of sterility and fertility. Mr. Darwin 

 has collected facts which go far to prove that colour (as 

 in the case of the black pigs of Virginia, which I cited 

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