SUBDIVISION OF MAMMALIA INTO ORDERS. 127 



the beauty of these animals ; thus the frequent rubbing, the use 

 of coverings, the precaution of washing and drying the extremi- 

 ties, and even of surrounding them with bands of flannel, are 

 circumstances which contribute powerfully to give to the horses 

 of our own country that neatness, which is remarked in the lower 

 part of their limbs, and to render their skin and their hair so 

 soft and fine. 



114. Thus, by modifying the circumstances in which an 

 animal is placed, we impress certain modifications on its struc- 

 ture ; and by employing for the propagation of the race only 

 the individuals thus modified, Man gives to the entire race a 

 particular character, and developes in it qualities which it did 

 not at first possess. It is probably in this way that he has 

 obtained the various races of dogs, whose forms are so different, 

 that at first sight we can hardly believe that they belong to one 

 and the same species. But this modifying power is always 

 restrained within certain limits ; and it never effaces the dis- 

 tinctive marks, which serve as the zoological characters of the 

 species. 



115. Classification of Mammalia. There exist, as we have 

 already seen, considerable differences amongst the Mammalia ; 

 and these modifications serve as a basis for the division of this 

 class into groups of an inferior rank, named Orders. The greater 

 part of these groups are so clearly separated from the surrounding 

 ones, that there can be no doubt regarding their limits ; and all 

 Zoologists have agreed to admit them as forming such natural 

 divisions. But in others the principal type is so modified, that 

 an almost insensible passage is made from the one to the other, 

 and it becomes very difficult to draw the line of division. A 

 certain Mammal, for example, has quite as much analogy with 

 the type which represents the order Quadrumana, as with that 

 of the Rodentia; and it may with nearly as much reason be 

 placed in the one as in the other of these divisions. 



116. It has been elsewhere explained ( ANIM. PHYSIOL. 750), 

 that the essential peculiarity which distinguishes the reproduc- 

 tive process in the Mammalia, is not so much that the egg is 

 retained within the body of the parent, so that the young is born 



