CAUSES OF THE STERILITY 315 



crosses. Mr. Salter has recently given the results of an ex- 

 amination of about 500 eggs produced from various crosses 

 between three species of Gallus and their hybrids; the ma- 

 jority of these eggs had been fertilised; and in the majority 

 of the fertilised eggs, the embryos had either been partially 

 developed and had then perished, or had become nearly ma- 

 ture, but the young chickens had been unable to break through 

 the shells. 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 chickens were 

 reared. With plants, hybridised embpyos 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 cir- 

 cumstanced 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 nour- 

 ished within its mother's womb, or within the egg or seed 

 produced by the mother, be exposed to conditions in some de- 

 gree 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 af- 

 ter 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 



