20S THE NATURALIST IN NICARAGUA. [Cla. XL 



crossed with another form, the tie that bound it to the 

 central stock is loosened, and the foundation is laid for 

 the formation of a new species. Further divergence 

 would be unchecked, or only slightly checked, and the 

 elements of reproduction having begun to vary, would 

 probably continue to diverge from, the parent form, for 

 Darwin has shown that any organ in which a species 

 has begun to vary, is liable to further change in the 

 same direction.* Thus one of the best tests of the 

 specific difference of two allied forms living together, 

 is their sterility when crossed, and nearly allied species 

 separated by geographical barriers are more likely to 

 interbreed than those inhabiting the same area. Ar- 

 tificial selection is more rapid in its results, but less 

 stable than that of nature, because the barriers that man 

 raises to prevent intermingling of varieties are tempo- 

 rary and partial, whilst that which nature fixes when 

 sterility arises is permanent and complete. 



For these reasons I think that the fact that the hair- 

 less dog of tropical America has not interbred with the 

 common form, and regained its hairy coat, is in favour of 

 the inference that the variety has been produced by 

 natural and not by artificial selection. By this I do not 

 mean that it has arisen as a wild variety, for it is 

 probable that its domestication was an important ele- 

 ment amongst the causes that led to its formation, but 



o 



that it has not been produced by man selecting the 

 individuals to breed from that had the least covering of 

 hairs. I cannot agree with some eminent naturalists 

 that the loss of a hairy covering would always be disacl- 



fr See "Animals and Plants under Domestication," vol. ii. p. 241. 



