264 HYBRIDISM. Chap. Tin. 
having a pistil too long for the pollen-tubes to reach the 
ovarium. It has also been observed that when pollen 
of one species is placed on the stigma of a distantly 
allied species, though the pollen-tubes protrude, they 
do not penetrate the stigmatic surface. Again, the 
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 suflSciently 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 this 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 
