INTRODUCTION 5 



pounds. But no animal can do this. An animal supplied 

 with the substances suitable for plant food would starve and 

 perish from inanition as quickly as if they were withheld from 

 it. It must have food already elaborated, in the first instance 

 through the agency of plants. It requires, in addition to 

 water, proteid or albuminous substance whether derived from 

 vegetable or animal sources, starches and sugars, and fats. 

 These are solid substances which the animal takes into itself, 

 or, as we say, ingests, and elaborates within its body into the 

 more complex substance of living tissue. 



It is by reason of this fundamental difference in nutrition 

 that plants and animals differ so widely from one another in 

 structure. The plant, dependent on gases and liquids for its 

 food, seeks to expose as much surface as possible to these 

 media. It spread its leaves in the air, and thus presents a 

 great surface for the assimilation of carbon from the carbon 

 dioxide of the air. Its roots penetrating into the earth seek 

 out the water which, percolating through the soil, has 

 dissolved and holds in solution the mineral matters necessary 

 to its sustenance. Fixed thus in the earth the plant is 

 independent of locomotion, of sense organs, of means of 

 capturing and swallowing its food, and, though it is true that 

 some of the lowest plants are locomotory, the proposition just 

 stated holds good for the vast majority of the members of the 

 vegetable kingdom. 



Animals, on the other hand, are under the necessity of 

 moving in search of their food. They require the means of 

 recognising it, of seizing and swallowing it, and since they do 

 take it into themselves or swallow it, they require some sort of 

 cavity or receptacle in which it may be contained and further 

 elaborated. Clearly, then, the relations of an animal to its 

 environment are far more complex than those of a plant ; and 

 they are the more complicated because animals stand in every 

 kind of relation to one another, and are endowed, as the case 

 may be, with special powers of offence or defence, with means 

 of protection, concealment, attraction, and even of deception. 



Looking at an animal from a general point of view, and 

 studying its necessities to some extent in the light of our own, 

 we may enumerate the several activities indispensable to its 

 existence. 



Each of these activities is effected, in higher animals like 



