Chap. IX.] OF FIRST CROSSKS AmU OF HyBRlDS. ^3 



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 na- 

 ture of the differences between species which induce 

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

 first crosses, the greater or less diificulty 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 spe- 

 cies 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 

 jvhy 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 communi- 

 cated 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 eitlier been partially developed and had then per- 



