Chap. IX.] OF FIEST CROSSES AND OF HYBRIDS. 28 



out the animal and vegetable kingdoms, we may infer 

 that the cause, whatever it may be, is the same or 

 nearly the same in all cases. 



We will now look a little closer at the probable 

 nature of the differences between species which induce 

 sterility in first crosses and in hybrids. In the case of 

 first crosses, the greater or less difficulty in effecting an 

 union and in obtaining offspring apparently depends on 

 several distinct causes. There must sometimes be a 

 physical impossibility in the male element reaching 

 the ovule, as would be the case with a plant having a 

 pistil too long for the pollen-tubes to reach the ovarium. 

 It has also been observed that when the 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 gi\ en 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 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 



