GENERAL INTEODDCTION. IxxXV 



harvested may produce an indifferent crop, so ma}- imma- 

 ture germs produce indifferent offspring. To summarise, 

 I believe experiments may sliow that the difference 

 between members of the same family, and members of the 

 same brood or litter, is mainly due to the structure and 

 condition of the germ-cells, and to the nature of the com- 

 binations formed when the carriers of heredity, the proto- 

 plasmic units derived from two separate individuals (the 

 parents), actually combine to form the new cell out of 

 which the new individual is gradually developed. 



STERILITY IN EQUINE HYBRIDS. 



It is not in obedience to any natural law that hybrids 

 are, as a rule, sterile. Formerly zoologists were ever 

 ready to consider intersterility as strong evidence of 

 specific distinction, even when the mutually sterile iorms 

 were almost identical in structure and habits. Wei-e 

 structure alone taken as the standard, not a few of the 

 animals that are looked upon as mere varieties would be 

 raised to the rank of species, while a number of species 

 would be degraded to the rank of varieties. Even as it 

 is, a number of species yield when crossed fertile off- 

 spring. For example, hybrids between the common goose 

 and the Chinese goose — two very distinct species — are 

 perfectly fertile, as are hybrids between the common duck 

 and the pintail duck, and between various sjDscies of 

 pheasants. The Indian buffalo and the American bison 

 produce fertile hybrids with the wild ox of Europe ; 

 while the common humped cattle of India yield fertile 

 hybrids with the domestic ox. 



Amongst plants, hybrids are sometimes quite fertile ; 

 while some crosses are quite, or almost, sterile. There 

 is no hard and fast line between species and varieties, and 

 heuce there can be no fundamental difference between a 

 hybrid and a cross, nor yet any a priori reason why any 

 given hybrid should be sterile, or any given cross fertile. 



It is no longer possible to contend that species were 

 originally endowed with mutual sterility by way of pre- 



