138 THE GENESIS OF SPECIES. [CHAP. 



It remains now to notice in favor of specific stability, 

 that the objection drawn from physiological difference be- 

 tween " species " and " races " still exists unrefuted. 



Mr. Darwin freely admits difficulties regarding the ste- 

 rility of different species when crossed, and shows satis- 

 factorily that it could never have arisen from the action of 

 " Natural Selection." He remarks 17 also : " With some few 

 exceptions, in the case of plants, domesticated varieties, 

 such as those of the dog, fowl, pigeon, several fruit-trees, 

 and culinary vegetables, which differ from each other in ex- 

 ternal characters more than many species, are perfectly fer- 

 tile when crossed, or even fertile in excess, while closely- 

 allied species are almost invariably in some degree sterile." 



Again, after speaking of " the general law of good being 

 derived from the intercrossing of distinct individuals of the 

 same species," and the evidence of the pollen of a distinct 

 variety or race is prepotent over a flower's own pollen, adds 

 the very significant remark, 18 " When distinct species are 

 crossed, the case is directly the reverse, for a plant's own 

 pollen is almost always prepotent over foreign pollen." 



Again he adds : " "I believe from observations commu- 

 nicated 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 five hundred eggs produced from various crosses 

 between three species of Gallus and their hybrids. The 

 majority of these eggs had been fertilized, and in the ma- 

 jority of the fertilized eggs the embryos either had been 

 partially developed and had then aborted, or had become 

 nearly mature, but the young chickens had been unable to 

 break through the shell. Of the chickens which were born, 



17 "Animals and Plants under Domestication," vol. ii., p. 189. 

 is "Origin of Species," 6th edit., 1869, p. 115. 

 19 Ibid., p. 322. 



