LECTURE XIV. . 421 



to the same race is impossible. As little as can the wolf copu- 

 late with the Fennec of the Sahara, as little can a bulldog pair 

 with the small hairless African or the Bolognese lap-dog : it 

 is a physical impossibility. Breeders are, moreover, aware, 

 that certain races can only be made to pair with the greatest 

 difficulty, that the prolificacy of the mongrels soon diminishes, 

 and that the race becomes extinct, whilst other races pair 

 readily and are prolific. " There are qualities," says Nathu- 

 sius, " which are incompatible ; hence every crossing does 

 not lead to a fusion of characters." There are according-ly 

 crossings which can never become constant. In other words, 

 there are races which are, but with difficulty, productive be- 

 tween each other, and there are others in which the fertility is 

 limited to a few generations. 



The aversion existing between alhed species in the wild 

 state tas also been considered of importance. We have 

 seen that this aversion is frequently overcome, especially by the 

 males ; but we know also that it increases with the difference 

 of the race. Eengger states expressly, that the cats imported 

 into Paraguay, which are now essentially changed, but whose 

 importation is historically proved, have a decided aversion to 

 European cats, and can only with difficulty be brought to pair 

 with them. " Birds of a feather flock together" is an old pro- 

 verb, and applicable to the whole animal world. It appears to 

 me highly probable that were the Swytz and the Saanen races 

 of our common cattle set at liberty they would not intermix ; 

 but that each race would select its own grazing districts, and 

 not intrude upon those of the other race. Possibly, the larger 

 Saanen race may supersede the weaker Swytz race, but no 

 voluntary intermixture would take place. 



On summarising the facts obtained in this field of inquirv, 

 we are justified in concluding that, as regards production and 

 propagation, there exists no difference whatever between species 

 and races ; that there are both races and species unable to pro- 

 pagate inter se ; that there are some which only propagate 

 with difficulty ; and again others which easily produce prohfic 

 hybrids, and thus give rise to new species and races. Expe- 

 rience, however, also teaches, that races and species intermix 



