238 PROBLEMS OF GENETICS 



much more critical data I suppose no one would nowadays be 

 inclined to follow Darwin in instituting a comparison between 

 the sterility of hybrids and that of illegitimately raised plants of 

 heterostyle species. 4 It is even difficult to imagine any essential 

 resemblance between these two phenomena, nor has evidence 

 ever been produced to show that illegitimately raised plants 

 have bad pollen grains, which is the usual symptom of sterility 

 in hybrid plants and the consequence, as we believe, of failure 

 of some essential division in the process of maturation. 



The difficulty that we have no knowledge of the contemporary 

 origin of forms, from a common stock, which when crossed together 

 give a sterile product, is one of the objections constantly and 

 prominently adduced from the time of the first promulgation of 

 evolutionary ideas. In the light of recent work the objection 

 has gathered strength. Why, if we are able to produce instances 

 of variation colourably simulating specific difference in almost 

 all other respects, do we never find an original appearance of this 

 most widely spread of all specific characteristics? No doubt all 

 breeders know that sterile animals and plants occasionally appear 

 in their cultures, but it is more in accordance with probability 

 that the sterility in these sporadic instances should be regarded 

 as due to defect than that it should be thought comparable 

 with that of the sterile hybrids. For their sterility must, by all 

 analogy with results elsewhere seen, be attributed not to the 

 absence of something, but to the presence and operation of 

 complementary factors leading to the production of inhibition 

 of division; and consistently with that interpretation, we find 

 that when from a partially sterile hybrid comparatively fertile 

 offspring can be raised, their comparative fertility continues in 

 the posterity generally if not always without diminution. The 

 distinction between these several kinds of sterility was of course 

 not understood in Darwin's time. The comparison, for example, 

 which he instituted 5 between the sterility of " contabescent " 

 anthers and that of hybrids no longer holds, for at least in those 

 cases in which the nature of contabescent anthers have been 

 genetically investigated (Sweet Pea, Tropaeolum) they proved 



4 Animals and Plants, ed. i, 1868, II, pp. 180-5. 

 B Animals and Plants, ed. i, 1868, II, p. 165. 



