346 FUNCTIONS OF 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 ; 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 
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