124 HEREDITY AND SELECTION IN SOCIOLOGY 



plant of one family is placed on the stigma of a plant of a distinct 

 family, it exerts no more influence than so much inorganic dust. 

 From this absolute zero of fertility, the pollen of different species 

 applied to the stigma of some one species of the same genus 

 yields a perfect gradation in the number of seeds produced up 

 to nearly complete, or even quite complete, fertility ; and, as we 

 have seen, in certain abnormal cases, even to an excess of fer- 

 tility beyond that which the plant's own pollen produces. So 

 in hybrids themselves there are some which never have produced, 

 and probably never would produce, even with the pollen of the 

 pure parents, a single fertile seed. From this extreme degree of 

 sterility, we have self-fertilised hybrids producing a greater and 

 greater number of seeds up to perfect fertility.^ 



Thus, it cannot be maintained that hybridism necessarily 

 entails sterility. But it is none the less certain that, as a general 

 rule, sterility does accompany hybridism, if not in the first, then 

 in subsequent generations. Crossing between distinct species 

 may be said to entail degeneracy and sterility as a general rule ; 

 this rule, like all others, has its exceptions, as we have shown, 

 but these comparatively rare exceptions do not affect the validity 

 of the general rule. 



And this degeneracy of hybrids is not surprising when we con- 

 sider that they are the product of the unnatural crossing of two 

 distinct species ; consequently, they are not in natural conditions : 

 there is a lack of harmony between them and their enviroiunent. 

 Their entire organisation has been disturbed by the mingling of 

 two distinct structures and constitutions, including the repro- 

 ductive systems. The only cause for surprise is, not that idti- 

 mate sterihty is the result of so incompatible a mixture, but that 

 immediate and unconditional sterihty does not result. 



As to the cause which directly renders either first crosses or 

 their products sterile, it may either lie in the physical impossi- 

 bility of the male element reaching the female element, or of the 

 1 The Origin of Species, p. 376. 



