264 HYBRIDISM. Chap. VIII. 



male element may reach the female element, but be 

 incapable of causing an embryo to be developed, as 

 seems to have been the case with some of Thuret's 

 experiments on Fuci. No explanation can be given 

 of these facts, any more than why certain trees cannot 

 be grafted on 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 

 gallinaceous birds, that the early death of the embryo 

 is a very frequent cause of sterility in first crosses. I 

 was at first very unwilling to believe in tins view; 

 as 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 coun- 

 try where their two parents can live, they are gene- 

 rally placed under suitable conditions of life. But a 

 hybrid partakes of only half of the nature and consti- 

 tution of its mother, and therefore before birth, as long 

 as it is nourished within its mother's womb or within 

 the egg or seed produced by the mother, it may 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 seem 

 eminently sensitive to injurious or unnatural condi- 

 tions of life. 



In regard to the sterility of hybrids, in which the 

 sexual elements are imperfectly developed, the case is 

 very different. I have more than once alluded to a 

 large body of facts, which I have collected, showing 

 that when animals and plants are removed from their 

 natural conditions, they are extremely liable to have their 

 reproductive systems seriously affected. Tins, in fact, is 



