242 PROBLEMS OF GENETICS 



nent uncertainty. But the experience of the practical breeder 

 does, I think, on the whole, support the contention to which 

 systematists have so steadily clung under all the assaults of 

 evolutionary philosophers, that, though we cannot strictly define 

 species, they yet have properties which varieties have not, and 

 that the distinction is not merely a matter of degree. 



The first step is to discover the nature of the factors which by 

 their complementary action inhibit the critical divisions and so 

 cause the sterility of the hybrid. Thus expressed, we see the 

 problem of inter-specific sterility in its right place; and the 

 question why we do not now find contemporary instances of 

 varieties lately arisen in domestication, which when crossed back 

 with their parents, or with their coderivatives, can produce 

 sterile products, is perceived to be only a special case of a problem 

 which in its more general form is that of the origin of new and 

 additional factors. 



For the requisite evidence no comprehensive search has been 

 made, but perhaps it will yet be found. All that we can say at 

 the present time is that the incidence both of hybrid sterility, 

 and of incompatibility also, is most capricious; and provided 

 that two forms have such features in common that a cross between 

 them seems not altogether out of the question, no one can predict 

 without experiment whether such a cross is feasible, and if 

 feasible whether the product will be fertile, or sterile more or 

 less completely. For instance, though probably all the British 

 and some Foreign Finches (Fringillidae) have been crossed 

 together, and some of these crosses, as for instance, the various 

 Canary-mules have been made in thousands, I believe no quite 

 clear example of a fertile hybrid can be produced. Many species 

 of Anatidae cross readily and produce fertile hybrids : others give 

 results uniformly sterile. Though most of the Equidae can be 

 crossed and some of the hybrids are among the commonest of 

 domesticated animals there is no certain record of a fertile mule. 

 Among the Canidae the dogs, wolves and jackals all give fertile 

 hybrids, but there is no clearly authenticated instance of a cross 

 between any of these forms and the European fox. In spite of 

 their close anatomical resemblance it is doubtful if the rabbit 



