24 CAUSES OF THE STERILITY [Chap. IX. 



eggs, the erabrvos 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, 

 hybridised embryos 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 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 fertilised, pass 

 through their early stages of development and then 

 perish like the embryos produced by a cross between 

 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 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 vathin 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 



