24 CAUSES OF THE STEHILlT? [Chap. IX, 



isfiJcl;- ut^-iad. 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, " with- 

 out any obvious cause, apparently from mere inability 

 to live; " so that from the 500 eggs only twelve chick- 

 ens were reared. With plants, hybridised embryos prob- 

 ably often perish in a like manner; at least it is known 

 that hybrids raised from very distinct species are some- 

 times 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 pro- 

 duced by a cross between distinct species. Until becom- 

 ing acquainted with these facts, I was unwilling to be- 

 lieve 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. Hy- 

 brids, 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 par- 

 takes 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 condi- 

 tions 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 



