IxXXviii GENERAL INTEODUCTION. 



been known to breed, zebras never seem to reach maturity 

 until the fifth or sixth year. Hence, though zebra hy- 

 brids are not fertile as two-year-olds, they may prove 

 fertile when three or four years old. If one may judge 

 by the behaviour of the male germ-cells in other mam- 

 mals, it is possible that a change of the surroundings — 

 of the food, temperature, &c. — might make a profound 

 difference, transform a sterile hybrid into a fertile one. 



Domestication is known to hurry on maturity. In a 

 wild state it would be a disadvantage for a young 

 stallion to reach maturity much before his full strength 

 is required, until be has some chance of successfully 

 challenging the old leader of his troop, or forming for 

 himself a new one. It may safely be taken for granted 

 that the wild ancestors of our horses were later in 

 reaching maturity than their pampered domesticated 

 descendants. Hence, apart from any retarding influence 

 the zebra may have had, Eomulus, in as far as he has 

 regressed towards his maternal ancestors, will presumably 

 be late in reaching maturity. 



In addition to Romulus, I have for some time had a 

 male zebra-ass hybrid under observation. This hybrid 

 having unfortunately died, I shall elsewhere be able to 

 describe at length the condition of the germ glands. In 

 the meantime I may, however, say that, as in Romulus, 

 the reproductive system of the three-year-old zebra-ass 

 hybrid seemed quite perfect, but, as in Romulus, the 

 germ-cells were incompletely developed, and hardly at 

 all mobile, owing to the tail being only on an average 

 twice the length of the head ; in the horse and zebra 

 the tail is quite eight times the length of the head. 

 This condition of the luale germ-cells is better described 

 as arrested development than as regression ; they closely 

 resemble a phase in the development of the corresponding 

 cells in the horse, but do not necessarily reproduce the 

 mature germ-cells in any of the ancestors of the Equid^e. 

 Whether, had this hybrid survived, ripe germ-cells would 

 have eventually appeared I am unable to say, and I am 

 also unable to say whether a different diet or different 



