300 EVOLUTION, GENETICS, AND EUGENICS 



zygosity within a species and between one species and another. More- 

 over, sexual reproduction as an adaptation operates chiefly through 

 bringing together a variety of combinations of characters possessed 

 by strains genetically diverse; it is, in fact, a hybridizing mechanism. 

 It seems probable therefore that hybridization as a factor in evolution 

 operates up to the limit of the adaptive possibilities inherent in its 

 mechanism. We may therefore conclude that hybridization is and has 

 been an important evolutionary factor, though we have at present 

 little information as to its precise mode of operation as an agent in 

 species formation. 



SOME ANIMAL HYBRIDS 



Onr of the arguments offered by the advocates of the theory that 

 hybridization has played a prominent part in species-forming in nature 

 is that man has carried on so much successful hybridization between 

 different species of animals and plants and has produced many very 

 useful hybrids, some of them better adapted than either of the parent 

 types from which the hybrids were derived. 



Especially interesting are animal hybrids produced by interbreed- 

 ing distinct species of domestic and wild animals. Our brief account 

 will be confined to horse, cattle, and fowl hybrids. 



Horse hybrids. — These hybrids are all the product of crossing dif- 

 ferent species of the genus Equus. The commonest hybrid is that be- 

 tween the mare of the horse proper, Equus caballus, and the jack of the 

 species E. hemionus. The result is the mule, a useful domestic animal 

 combining some of the best features of the two species and superior to 

 either in vigor, hardiness, capacity for work, and resistance to disease. 

 The reciprocal cross, made by mating a jennet to a stallion, is called a 

 "hinny," a sort of small mule. Neither mules nor hinnies are, as a 

 rule, fertile, so they are not self-perpetuating. The rare reports of 

 female mules giving birth to foals, when bred to jacks, are as yet not 

 fully authenticated. There is some possibility that the particular fe- 

 males in question were either rather large jennets, which of course 

 would be fertile, or rather mulelike mares. 



The sterility of mules has been shown by Wodsedalek to be due to 

 differences in the numbers of chromosomes in the two species, the horse 

 having in the female 18 autosomes and 2 X-chromosomes, the male 18 

 autosomes and one X-chromosome; the ass having in the female 32 

 autosomes and 2 X-chromosomes, the male 32 autosomes and (accord- 

 ing to Paynter) an X- and a Y-chromosome. With this great discrep- 

 ancy in chromosomes, it is hardly to be expected that meiosis could 

 operate so as to give any sort of viable gametes or zygotes. 



