276 CAtfSES 6f Tim STfcRILlT? 



majority of the fertilized eggs, the embryos had either been 

 partially developed and had then perished, or had become 

 nearly mature, but the young chickens had been unable to 

 break through the shell. Of the chickens which were born, 

 more than four-fifths died within the first few days, or at 

 latest weeks, " without any obvious cause, apparently from 

 mere inability to live ; " so that from the 500 eggs only 

 twelve chickens were reared. With plants, hybridized em- 

 bryos probably often perish in a like manner ; at least it is 

 known that hybrids raised from very distinct species are 

 sometimes weak and dwarfed, and perish at an early age ; 

 of which fact Max Wichura has recently given some strik- 

 ing 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 dis- 

 tinct species. Until becoming acquainted with these facts, 

 I was unwilling to believe in the frequent 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 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 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 conditions of life. But 

 after all, the cause more probably 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 subsequently exposed. 



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

 elements 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, 



