HYBRIDISM 29 



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

 first crosses, the greater or less difficulty in effecting 

 a 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 

 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 sufii- 

 ciently attended to; but I believe, from observations 

 communicated to me by Mr. Hewitt, who has had great 

 experience in hybridizing 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 fertil- 

 ized, and in the majority of the fertilized eggs the em- 

 bryos 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, "with- 



