Chap. ix. Degi'ees of Sterility. 241 



was first propounded by Pallas, seems by far the most probable, and 

 can, indeed, hardly be doubted. It is, for instance, almost certain 

 that our dogs are descended from several wild stocks ; yet, with 

 perhaps the exception of certain indigenous domestic dogs of South 

 America, all are quite fertile together; but analogy makes me 

 greatly doubt, whether the several aboriginal species would at first 

 have freely bred together and have produced quite fertile hybrids. 

 So again I have lately acquired decisive evidence that the 

 crossed offspring from the Indian humped and common cattle are 

 inter se perfectly fertile ; and from the observations by Riitimeyer 

 on their important osteological differences, as well as from those 

 by Mr. Blyth on their differences in habits, voice, constitution, &c, 

 -these two forms must be regarded as good and distinct species. 

 The same remarks may be extended to the two chief races of 

 the pig. We must, therefore, either give up the belief of the 

 universal sterility of species when crossed; or we must look at 

 this sterility in animals, not as an indelible characteristic, but 

 ;as one capable of Joeing removed by domestication. 



Finally, considering all the ascertained facts on the intercross- 

 ing of plants and animals, it may be concluded that some degree 

 of sterility, both in first crosses and in hybrids, is an extremely 

 general result ; but that it cannot, under our present state of 

 knowledge, be considered as absolutely universal. 



Laws governing the Sterility of first Crosses and of Hybrids. 



We will now consider a little more in detail the laws governing 

 the sterility of first crosses and of hybrids. Our chief object will 

 be to see whether or not these laws indicate that species have 

 been specially endowed with this quality, in order to prevent 

 their crossing and blending together in utter confusion. The fol- 

 lowing conclusions are drawn up chiefly from Gartner's admirable 

 work on the hybridisation of plants. I have taken much pains 

 to ascertain how far they apply to animals, and, considering how 

 scanty our knowledge is in regard to hybrid animals, I have 

 been surprised to find how generally the same rules apply to both 

 kingdoms. 



It has been already remarked, that the degree of fertility, both 

 •of first crosses and of hybrids, graduates from zero to perfect 

 fertility. It is surprising in how many curious ways this grada- 

 tion can be shown ; but only the barest outline of the facts can 

 here be given. When pollen from a plant of one family is placed 

 on the stigma of a plant of a distinct family, it exerts no more 

 influence than so much inorganic dust. From this absolute zero of 



