HYBRIDISM . 253 



of natural selection. Both Gartner and Kolreuter have proved 

 that in genera including numerous species, a series can be formed 

 from species which when crossed yield fewer and fewer seeds, to 

 species which never produce a single seed, but yet are affected 

 by the pollen of certain other species, for the germen swells. It is 

 here manifestly impossible to select the more sterile individuals, 

 which have already ceased to yield seeds; so that this acme of 

 sterility, when the germen alone is affected, cannot have been 

 gained through selection ; and from the laws governing the various 

 grades of sterility being so uniform throughout 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 ap- 

 parently depends on several distinct causes. There must some- 

 times 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 ob- 

 served 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 caus- 

 ing 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 com- 

 municated 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 

 fertilized; and in the majority of the fertilized eggs, the embryos 

 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, 

 ^'without any obvious cause, apparently from mere inability to 



