GENERAL INTRODUCTION. IxXXvii 



a different kind of food. In discussing sterility, it is 

 generally pointed out that of all the systems the repro- 

 ductive system is most sensitive to changes in the sur- 

 roundings. 



If the reproductive system is especially sensitive, if, 

 e. g., by inappreciable changes in the surroundings fer- 

 tility is increased, impaired, or completely lost, need we 

 be surprised that hybrids (in which the reproductive 

 system is of necessity modified by regressive and other 

 changes) are frequently sterile ? 



But it will be asked. What evidence is there that the 

 reproductive system of hybrids in any way differs from 

 that of the parent species ? Strange as it may seem, 

 there does not yet appear to exist an account of the re- 

 productive system of either mules or hinnies, or, as far as 

 I can discover, of any mammalian hybrids. Female 

 mules may, as is now and then alleged, be fertile, while 

 male mules may be invariably sterile, but until the germ- 

 forming glands of both male and female mules and hinnies 

 have been systematically investigated it is hardly safe 

 to hazard an opinion on the subject in the absence of 

 well-authenticated cases of fertility. I have not yet had 

 an opportunity of examining either the germ-forming 

 glands of ordinary mules or hinnies, but I have recently 

 had the chance of learning something of the reproductive 

 glands of zebi'a hybrids. 



Sterility of Male Zebra Hyhrids. — In the two-year-old 

 hybrid, Eomulus, the reproductive appai-atus seems to be 

 fully developed, as are the reproductive instincts. As in 

 his zebra sire, there are large teats, but, unlike both sire 

 and dam, though doubtless like the ancestors of both, the 

 skin forming the scrotal sac has a complete coating of 

 hair ; this is doubtless another instance of regression. 



But notwithstanding the fact that Eomulus is apparently 

 as well developed as a horse of the same age, he has not 

 yet reached maturity, i. e. his germ-cells are still incom- 

 pletely formed ; there is a head, but the merest rudiment 

 of a tail. This may, however, be accounted for by his 

 mixed origin. Though yearling colts and fillies have 



