254 THE ORIGIN OF SPECIES 



live;" so that from the 500 eggs only twelve chickens were reared. 

 With plants, hybridized embryos probably often perish in a like 

 manner; at least it is known that hybrids raised from very dis- 

 tinct species are sometimes weak and dwarfed, and perish at an 

 early age; of which fact Max Wichura has recently given some 

 striking cases with hybrid willows. It may be here worth noticing 

 that in some cases of parthenogenesis, the embryos within the 

 eggs of silk moths which had not been fertilized, pass through their 

 early stages of development and then perish like the embryos 

 produced by a cross between distinct species. Until becoming ac- 

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

 quent early death of hybrid embryos; for hybrids, when once 

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

 of the common mule. Hybrids, however, are differently circum- 

 stanced 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 hybrid partakes of only half 

 of the nature and constitution of its mother; it may therefore, 

 before birth, as long as it is nourished within its mother's womb, 

 or within the egg or seed produced by the mother, be exposed to 

 conditions in some degree unsuitable, and consequently be liable 

 to perish at an early period; more especially as all very young 

 beings are eminently sensitive to injurious or unnatural condi- 

 tions of life. But after all, the cause more probably lies in some 

 imperfection in the original act of impregnation, causing the em- 

 bryo to be imperfectly developed, rather than in the conditions to 

 which it is subsequently exposed. 



In regard to the sterility of hybrids, in which the sexual ele- 

 ments are imperfectly developed, the case is somewhat different. 

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

 that, when animals and plants are removed from their natural 

 conditions, they are extremely liable to have their reproductive 

 systems seriously affected. This, in fact, is the great bar to the 

 domestication of animals. Between the sterility thus superinduced 

 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 various 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 

 systematic affinity, for whole groups of animals and plants are 

 rendered impotent by the same unnatural conditions; and whole 

 groups of species tend to produce sterile hybrids. On the other 



