346 FUNCTIONS OP ORGANIC AND ANIMAL LIFE. 



circulates, in order to drive it through them with the requisite 

 certainty and energy. The respiration of Animals, again, is 

 essentially the same with that of Plants j the chief difference 

 being that, in order to secure the active performance of this 

 important function, the higher Animals are provided with 

 a complex apparatus of nerves and muscles, by which the air 

 or water in contact with the aerating surface is continually 

 renewed. And in regard to the functions of secretion and 

 excretion, we have seen that, though there is a wide difference 

 in the form of the organs by which they are executed, they 

 are the same in essential structure ; and that the difference in 

 their mode of operation consists chiefly in this, that their 

 products in the Animal are destined to be carried out of the 

 body, instead of being retained within it, as in Plants. 



426. In regard to the immediate objects of these functions, 

 also, there is but little essential difference ; for in both in- 

 stances it is the conversion of alimentary materials into living 

 organized tissue. But the ultimate purpose of this tissue is 

 far from being the same in the two kingdoms. Nearly all the 

 nourishment taken-in by Plants is applied to the extension of 

 their own fabric ; and hence there is scarcely any limit to the 

 size they may attain. There is very little waste or decay of 

 structure in them, the parts once formed (with the exception 

 of the leaves and flowers) continuing to exist for an indefinite 

 time ; this is a consequence of the simply physical nature of 

 the functions of the woody structure, which has for its chief 

 object to give support to the softer parts, and to serve as the 

 channel for the movement of the fluid that passes towards and 

 from them. The case is very different in regard to Animals. 

 With the exception of those inert tribes which may be com- 

 pared with Plants in their mode of life, we find that the 

 whole structure is formed for motion ; and that every act of 

 motion involves a waste or decay of the fabric which executes 

 it. An energetic performance of the nutritive actions is re- 

 quired, therefore, in the more active Animals, simply to make 

 good the loss which thus takes place ; we find, too, that their 

 size is restrained within certain limits ; so that, instead of the 

 nourishment taken into the body being applied, as in Plants, 

 to the formation of new parts, it is employed for the most part 

 in the simple repair of the old. Thus we may say that, whilst 

 the ultimate object of Vegetable Life is to build up a vast 



