162 The Naturalist in Nicaragua 
As long as varieties interbreed together and with the parent 
form, it does not seem possible that a new species could be 
formed by natural selection, excepting in cases of geographical 
isolation. All the individuals might vary in some one 
direction, but they could not split up into distinct species 
whilst they occupied the same area and interbred without: 
difficulty. Before a variety can become permanent, it must 
be either separated from the others or have acquired some 
disinclination or inability to interbreed with them. So long 
as they interbreed together, the possible divergence is kept 
within narrow limits, but whenever a variety is produced, 
the individuals of which have a partiality for interbreeding, 
and some amount of sterility when crossed with the parent 
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 lable to further change in the same 
direction.1 Thus one of the best tests of the specific dif- 
ference of two allied forms living together is their sterility 
when crossed, and nearly allied species separated by geo- 
graphical barriers are more likely to interbreed than those 
inhabiting the same area. Artificial 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 temporary and partial, whilst that which 
nature fixes when sterility arises is permanent and complete. 
For these reasons I think that the fact that the hairless 
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 element amongst the causes that led to its 
formation, but that it has not been produced by man select- 
ing 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 disadvan- 
1 See Animals and Plants under Domestication, vol. ii. p. 241. 
