HISTORY OF ZOOLOGY. 35 



Hybrids of hares and rabbits have continued fruitful for 

 generations; the same is true of hybrids obtained from 

 the wild buck and the domesticated she-goat; from Anscr 

 cygnoidcs and Anscr doincsticus ; from Salmo salvelimts and 

 ,S. fontinalis ; Cyprinns carpio and Carassius vulgar is; 

 BoDibyx cynthia and B. arrindia. 



D. Inbreeding. Even the second of the above state- 

 ments, that individuals of a species, provided they are sound, 

 always reproduce with one another, needs limitation. Breed- 

 ers of animals have for a long time known the disastrous 

 consequences of inbreeding that the reproductive power 

 is reduced even to sterility if, for breeding, descendants of 

 a single pair are continually chosen. Darwin has collected 

 not a few cases where undoubted members of the same 

 species have been completely sterile with one another; 

 thus certain forms of primrose and other cli- and tri- 

 morphic species. Examples of the sterility of mongrels 

 are known only in botany (certain varieties of maize and 

 mullein). 



Conditions Governing Fertility in Sexual Reproduc- 

 tion. When we look over these known facts it would seem 

 as if continued fertility in sexual reproduction were guaran- 

 teed by a not too considerable difference in the sexual prod- 

 ucts. Too great similarities, as these exist in inbreeding, 

 and too great differences, as in the hybridization of different 

 species, are injurious and are abhorred by Nature. Sexual 

 reproduction possesses an optimum ; if this is departed 

 from in either direction diminution gradually follows. 

 But for that reason it has already been said that here 

 gradual and not primary differences exist, and therefore 

 this character cannot be employed as a primary distinction 

 between species and varieties. 



Difficulties in Systemization. The final result of all 

 this discussion may be summed up as follows : up to the 

 present time, neither by physiological nor by morpho- 

 logical evidence has there been successfully fixed in a clear 

 and generally applicable way a criterion which can guide 

 the systematist in deciding whether certain series of forms 



