642 THE PHYSIOLOGY OF REPRODUCTION 



the cow and Bison honasus are similar.^ These and other observations 

 show that sterility among hybrids between closely allied species, 

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

 been recorded from among plants. 



Tiie 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, Iwanoff^ 

 states that hybrids between the horse and the zebra do not possess 

 spermatozoa, though able to perform the sexual act. 



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 chromo- 

 somes, we can readily imagine that any differences in their behaviour 

 at this time might lead to disastrous ^results." * 



It has been shown that irregular reduction divisions occur in the 

 male, and that in hybrid pheasants a degeneration of germ-cells 

 begins in synapsis. In some hybrids the number of chromosomes 

 derived from each parSnt is different, but as Babcock and Clausen 

 point out, the diiiiculty must be fundamentally physiological, .since it 

 is just as pronounced in hybrids between species having the sa,me 

 number of chromosomes. 



Inheritance . OF Feetiuty 



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 Blackfaced, are relatively infertile.^ 

 Furthermore, there is a considerable 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, 



' Iwanofif, " Sur la Fecondit6 de Bison bonasus x Bos taurus," C. R. de la Soc. 

 de Biol., vol. Ixxv., 1913. See also other papers by Iwanoff (C. R. de la Soc. de 

 Biol., vol. Ixx.,. 1911 ; Biol. Cent., vol. xxxi., 1911 ; and Zeitsch. f. ind. Abst. u. 

 Vererbungslehre, vol. xvi., 1916). 



^ See Suchetet, "Problfemes Hybridologiques," Jowr. de VAiiat. et la Pkys., 

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



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

 Biol. Cent., vol. xxv., 1905. "De la F6condation Artificielle chez les 

 Mammifferes," Arch, des Sciences Biologiques, vol. xii., 1907. 



* Morgan, Experimental Zoology, New York, 1907. See also Heredity and 

 Sex, New York, 191.3. 



* 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. 



