250 CAUSES OF THE STERILITY Ciixr. VIII 



and perish at an early aa;o ; of ■vvliicli fact Max Wirliura has 

 recently given some striking' cases with hybrid -willows. It 

 may be here worth noticing that in some cases of partheno- 

 genesis, embryos jiroduccd from the eggs of silk-motlis, which 

 liad not been fertilized, passed through their early stages of 

 development and then perished like the embryos produced by 

 a cross between two distinct species. Until becoming ac- 

 quainted with these facts, I was unwilling to believe in the fre- 

 fjuent early death of hybrid embryos ; for hj-brids, wlien once 

 born, are generally healthy and long-lived, as we see in the 

 case of the common mule. Hybrids, liowever, are differently 

 circumstanced before and after birth : when born and living in 

 a country where their two parents live, they are generally 

 placed under suitable conditions of life. But a hj-brid partakes 

 of only half of the nature and constitution of its mother, and 

 therefore before birth, as long as it is nourished within its 

 mother's womb, or within the egg or seed produced by the 

 mother, it may be exposed to conditions in some degree unsuit- 

 able, and consequently be liable to perish at an early period; 

 more especially as all very 3-oung beings are eminently sensitive 

 to injurious or uTuiatural conditions of life. But, after all, the 

 cause more jirobal)!}' lies in some imperfection in the original 

 act of impregnation, causing the embryo to be imperfectly 

 developed, rather than in the conditions to which it is subse- 

 quently exposed. 



In regard to the sterility of hyl)rids, in whicli the sexual ele- 

 ments are imperfectly dcveloi)cd, the case is dilferent. I have 

 more than once alluded to a large body of facts, which I have 

 collected, showing that, when animals and plants are removed 

 from tlieir natin-al conditions, they are extremely liable to have 

 their reproductive svstems seriously affected. This, in fact, is 

 the great bar to the domestication of animals. Between the 

 sterility thus sujicrinduced and that of hybrids, there are many 

 points of similarity. In both cases the sterility is independent 

 of general health, and is often accompanied by excess of size 

 or great luxuriance. In both cases the sterility occurs in vari- 

 ous degrees ; in both, the male element is the most liable to 

 be affected ; but sometimes the female more than the male. In 

 both, the tendency goes to a certain extent with S3-stematic 

 affinity, for whole groups of animals and jilants are rendered 

 impotent by the same unnatural conditions; and whole groups 

 of species lend to produce sterik^ hvbrids. On the other hand, 

 one species in a grouji will sometimes resist great changes of 



