604 THE PHYSIOLOGY OF REPRODUCTION 



show that sterility among hybrids between closely allied species, 

 although usual, is very far from being universal. 1 Similar cases 

 have been recorded from among plants. 



The cause of sterility in hybrid organisms is still to a large 

 extent an open question. In some cases the generative organs 

 are atrophied or imperfectly developed, while in most, if not 

 all sterile hybrids, the gametes are not developed. For example, 

 IwanofE 2 states that hybrids between the horse and the zebra 

 do not possess spermatozoa. 



It has been suggested that the sterility is due to irregularities 

 in the mechanics of division in the germ cells. " When we recall 

 that at one stage in the development of the germ cells there 

 may be a pairing and subsequent fusion of the maternal and 

 paternal chromosomes, we can readily imagine that any differ- 

 ences in their behaviour at this time might lead to disastrous 

 results." 3 



INHERITANCE OF FERTILITY 



That fertility is a racial characteristic, and consequently is 

 capable of hereditary transmission, is a fact that is generally 

 accepted. Among sheep, for example, some breeds, like the 

 Dorset Horns, the Hampshire Downs, and the Limestones, are 

 notoriously prolific, while other varieties, like the Scotch Black- 

 faced, are relatively infertile. 4 Furthermore, there is a con- 

 siderable amount of evidence that in each breed there are 

 particular strains of related individuals which have a higher 

 degree of fertility than the average, and that flockmasters, by 

 breeding from twin ewes and employing the services of twin rams, 

 have been able permanently to increase the fertility of their stock. 5 



1 See Suchetet, " Problemes Hybridologiques," Jour, de I'Anat. et la Phys., 

 vol. xxxiii., 1897 ; Dewar and Finn, The Making of Species, London, 1909. 



2 Iwanoff, " Untersuchungen iiber die Unfruchtbarkeit von Zebroiden," 

 Biol. Cent., vol. xxv., 1905. "De la Fecondation Artificielle chez les Mammi- 

 feres," Arch, dee Sciences Bioloyiques, vol. xii., 1907. 



3 Morgan, Experimental Zoology, New York, 1907. 



4 The Leicester breed of sheep is characterised by a relatively low fertility, 

 and this is said to be due to the preference that was shown to large single 

 lambs at the time when high prices ruled, and the consequent discarding 

 of ewes which bore twins. See Wallace (R.), loc. cit. 



8 Marshall, " Fertility in Scottish Sheep," Trans. Highland and Agric. 

 Soc., vol. xx., 1908. 



