Various Modifying Agencies. 141 



none. The Bears appear to be an exception, for although they are 

 placed among the carnaria, in Cuvier's classification, their dentition is 

 almost frugivorous, and the preferences of most of the species are for 

 wild fruits with honey, &c. When these fail they will catch fish or 

 small mammals, such as pigs. Even the Polar Bear, which habitually 

 lives on fish and seals, can be made to subsist on a vegetable diet. Yet 

 these plantigrade animals, in common with the insectivorous planti- 

 grades (Hedgehogs, Shrews, Tenrecs, &c.) and the semi-plantigrade 

 Weasels and Martens, possess no coecum. 



These animals are perhaps derived from a carnivorous ancestry, and 

 have not as yet made a sufficiently radical change in their diet, or have 

 not been subject to the change long enough to work much change in 

 their alimentary system, or the change may have taken the form of an 

 increased length of intestine. Cuvier remarks that the length of the 

 intestine of the domestic cat is much greater than that of the wild cat, 

 obviously owing to the less carnivorous and concentrated nature of its diet. 



The subdivision of the stomach into several sacs appears to be supple- 

 mentary to an imperfect mastication, and is supplied to animals that 

 bolt their food without chewing it, as in the case of the ruminants, and 

 also the whale tribes who swallow their food " in oceans " and never 

 chew it. 



The indirect effect of food upon the animal, besides its influence on 

 the nervous system through the peculiarities of its various chemical re- 

 actions, whereby the mentality, temperament, &c. , are largely formed, 

 consists in developing the habits necessary for its procurement. If the 

 food has a habit of growing at the top of a tree, the animal must adopt 

 the habit of climbing after it. If the food grows at the surface of the 

 ground, the head of the animal must be bent down to reach it. If it is 

 under ground he must learn to dig for it, if under water, to dive for it. 

 All the ten thousand acts required from one or another to procure their 

 food, tend to modify, to a greater or less extent, the organs that are 

 brought into requisition for the purpose ; constantly improving them by 

 the practice that makes perfect, while heredity clinches every modifica- 

 tion that survives the test of utility and the competition of the struggle^ 

 for life. 



Head and Tail. 



Among the first and most obvious differentiations in the animal body, 

 as already observed, is that by which the inside is distinguished 

 from the outside. Next and equally obvious and automatic is that sub- 

 division of the outside by which, in all vertebrate animals at least, a top 

 and bottom become recognized as the dorsal and ventral sides ; flanking 

 these two other aspects become the right and left, and the two poles 

 become the fore end and hind end or head and tail. 



