CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 233 



others. Lastly, an embryo may be developed, and then perish at 

 an early period. This latter alternative has not been sufficiently 

 attended to; but I believe, from observations communicated to 

 me by Mr. Hewitt, who has had great experience in hybridising 

 pheasants and fowls, that the early death of the embryo is a very 

 frequent cause of sterility in first crosses. Mr. Salter has recently 

 given the results of an examination of about 500 eggs produced 

 from various crosses between three species of Gallus and their 

 hybrids; the majority 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 

 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 gener- 

 ally 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 con- 

 stitution 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 fn 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 anbnals and plants are removed from their natural 

 conditions, they are extremely liable to have their reproductive 



