294 



CA USES OF THE STERILITY 



the majority of these eggs had been fertilized; and iix the 

 majority of the fertilized eggs, the embryos had either been 

 partially developed and had then perished, or had become 

 neai'ly 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 em- 

 bryos 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 be- 

 tween distinct 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 cir- 

 cumstanced before and after birth: when born and living 

 in a country where their two parents live, they are gener- 

 ally 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 Qgg 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 impregna- 

 tion, 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 



