122 HEREDITY AND SELECTION IN SOCIOLOGY 



hybrids ; in the second place, there is no relation between the 

 difficulty of crossing species and their fertility, or between the 

 structural resemblance of species and their aptitude for crossing ; 

 in the third place, what is perhaps most remarkable, is that the 

 male of a certain species A may fertilise the female of a species B, 

 whereas the male of B is unable to fertiUse the female of A. 

 In a word, the fertility of what Darwin has called first crosses 

 between different species of the same genus, or even between 

 species belonging to difierent genera, is intrinsically variable, 

 and depends largely on the constitution of the individuals who 

 happen to make the experiment. A distinction must, however, 

 be made between the first cross of two individuals, each of which 

 belongs to a pure species, and the subsequent crossing of their 

 hybrid products. 



We have said that hybrids are not invariably sterile. Darwin, 

 who championed this view against the eminent botanists Kol- 

 reuter and Gartner, attributed the sterihty of hybrid plants in 

 many cases to too close interbreeding. " I have made so many 

 experiments and collected so many facts," he writes, " showing, 

 on the one hand, that an occasional cross with a distinct indi- 

 vidual or variety increases the vigOux and fertility of the offspring, 

 and, on the other hand, that very close interbreeding lessens 

 their vigour and fertility, that I cannot doubt the correctness of 

 this conclusion."'- This view of Darwin's is a logical corollary 

 of the theory which attributes to consanguinity a tendency to 

 accumvlate inheritance, to bring together into one germ-plasm an 

 ever-increasing number of homogeneous ids. If, as we have said, 

 a pathological predisposition already exists in a family, con- 

 sanguineous intermarriage spreads the determinants of this pre- 

 disposition among the whole family, and accumulates them until 

 sterility is the result. As hybridism involves, as far as the repro- 

 ductive organs are concerned, an undoubtedly pathological and 

 abnormal condition, the interbreeding of individuals in this 

 1 The Origin of Species, p. 369. 



