138 SECRETS OF EARTH AND SEA 



— will have no such difficulty in effecting the fertilization of 

 the native species as have those adjacent species against 

 whose intrusions the latter is specifically moulded and 

 selected by long generations of severe natural selection. 



The failure of hybrids generally to ripen their ova and 

 sperm so as to reproduce themselves is a subject upon 

 which, considering its enormous importance and signifi- 

 cance, singularly little has been done in the way of investi- 

 gation. Fifty years ago it was usually taught that the 

 mule, between the horse and the ass, so largely produced 

 under human superintendence for transport service, was 

 unable to breed owing to some deformity in the reproductive 

 passages. Even now no adequate study of the subject has 

 been made, but it appears that whilst a female mule can 

 be, and sometimes is, successfully mated to a horse or an 

 ass, giving birth to a foal, the male mule does not produce 

 fully-formed spermatozoa. What precisely is the nature 

 of this failure, what the ultimate microscopic condition of 

 the sperm cells in infertile male mules, or in any other infer- 

 tile male hybrids, has not yet been properly worked out 

 by modern cytological methods. It would be a matter of 

 vast interest to determine what is the difference in the 

 structure of the sperm-cells of a fertile and of an infertile 

 male hybrid. At present, so far as I know, this has not 

 been done. 



So far what I have written applies to hybridization — 

 the inter-breeding of distinct species. A similar but by 

 no means identical subject is that of the inter-breeding of 

 distinct races or varieties of one species, and the production 

 of " mongrels." " Mongrels " are to races what " hybrids " 

 are to species. To this branch of the subject belongs the 

 study of the effects of intermarriage between distinct 

 races of men. 



