OF FIRST CROSSES AND OF HYBRIDS. 275 



detail: for with plants we have conclusive evidence that 

 the sterility of crossed species must be due to some princi- 

 ple, quite independent of natural selection. Both Gartner 

 and Kolreuter have proved that in genera including numer- 

 ous 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 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 inca- 

 pable 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 suf- 

 ficiently attended to ; but I believe, from observations com- 

 municated to me by Mr. Hewitt, who has had great ex- 

 perience 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 j and in the 



