EXTENT OF VARIATION IN RACES OF THE SAME SPECIES. 79 



Thus, the Feline races, with a few exceptions, are fitted to maintain life only 

 in tropical climates, and very speedily die in colder countries (unless kept 

 warm by artificial means), in consequence of their deficiency of heat-pro- 

 ducing power, and the want of a close downy fur adapted to retain the caloric 

 generated in their bodies. On the other hand, the Dog is enabled to accom- 

 pany Man, in the coldest as well as the hottest regions of the globe ; his 

 power of generating heat being capable of variation, in accordance with the 

 external temperature ; and his entire organization undergoing modifications, 

 which adapt it to the change in the conditions of its existence. It appears, 

 then, that it is quite impossible to fix upon any difference of structural pecu- 

 liarities, as indications of the distinctness of species ; until it has been ascer- 

 tained by observation, whether they are constant and invariable, the races 

 neither exhibiting any tendency to change in successive generations, nor 

 showing any disposition to mutual approximation, by the occasional modifi- 

 cation of the distinctive characters in the individuals composing them. 



3. On the possible Extent of Variation within the Limits of Species. 



68. We now come to the second point of our inquiry, namely, the 

 amount of variation which may take place in races, historically known to 

 have had a common parentage. There is considerable difficulty in obtaining 

 the most complete evidence upon this subject ; owing to the want of accurate 

 observation in the more remote historical periods, when it is probable that 

 most of the varieties or breeds of our domesticated animals were first origi- 

 nated. Still there is an adequate amount of proof, that these races may 

 undergo very considerable modifications, in the course of a few generations ; 

 and that new races or breeds, distinguished by marked peculiarities, may 

 originate even at the present time. Our most satisfactory information is de- 

 rived from the changes, which have taken place in the races of domesticated 

 animals, introduced into the West Indies and South America, by the Span- 

 iards, three centuries since. Many of these races have multiplied exceed- 

 ingly, on a soil and under a climate congenial to their nature; and several of 

 them have run wild in the vast forests of America, and have lost all the most 

 obvious appearances of domestication. The wild tribes are found to differ 

 physically from the domesticated breeds, from which they are known to have 

 originated ; and there is good reason to regard this change, as a partial restora- 

 tion of the primitive characteristics of the wild stocks, from which the tamed 

 animals originally descended. Thus we find that the Hog, where it has re- 

 turned to its wild state, nearly resembles the Wild Boar, which has never 

 been in a state of domestication. The colour loses the variety found in the 

 domestic breeds ; the Wild Hogs of the American forests being uniformly 

 black. The thin covering of hair and scattered bristles is replaced by a thick 

 fur, often somewhat crisp ; beneath which is found, in those which inhabit 

 the colder regions, a species of wool. The head, too, becomes much larger 

 in these wild races, as in the original Boar; and the differences in the conform- 

 ation of the cranium, between these and the domesticated breeds, are fully 

 equal to anything that is seen in the human race. The variations which pre- 

 sent themselves in other races of domesticated animals introduced into South 

 America at the same period, such as the horse, ass, ox, sheep, goat, dog, 

 cat, and gallinaceous birds, are not less striking. Still more remarkable 

 variations are seen in certain domesticated breeds, which must without doubt 

 have sprung from the same stock with the ordinary ones, although their origin 

 cannot be traced historically ; thus, in some localities we find swine with 

 solid hoofs ; in others, the hoof is cleft into five parts ; and in others, again, 

 the toes are developed to a monstrous length. 



