CHAP. IX.] DEGREES OF STERILITY. 225 



profit, where neither pure parent-species exists, they must certainly 

 be highly or perfectly fertile. 



With our domesticated animals, the various races when crossed 

 together are quite fertile ; yet in many cases they are descended 

 from two or more wild species. From this fact we must conclude 

 either that the aboriginal parent-species at first produced perfectly 

 fertile hybrids, or that the hybrids subsequently reared under 

 domestication became quite fertile. This latter alternative, 

 which 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 abori- 

 ginal species would at first have freely bred together and have 

 produced quite fertile hybridu. 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 osteo- 

 logical 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 being 

 removed by domestication. 



Finally, considering all the ascertained facts on the inter- 

 crossing 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 

 following conclusions are drawn up chiefly from Gartner's ad- 

 mirable 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 



