INTRODUCTORY 



crest of the physiological wave, consisting of matter in a condition 

 of extremely unstable equilibrium . . ." (fig. 303). 



The size, shape, and structure of animals, their habits and 

 distribution, all very largely depend upon the ever-present necessity 

 for food, which indicates its existence by the cravings of hunger 

 .and thirst. And it is ^ photoplasm 



1 



o<'. 



^ 



y 



STORED 

 ENERGY 



Fig. 303. — Metabolic Staircase 



in the main the radical 

 difference as to food 

 which brings about 

 the striking contrasts 

 in form, structure, and 

 mode of life between 

 animals and plants, 

 ■especially ordinary green plants, as distinguished from fungi. The 

 food of such green plants is of the simplest kind, consisting, as it 

 •does, of the carbonic acid gas of the air, and the water which every- 

 where permeates the soil and holds mineral salts in solution. A 

 plant is therefore surrounded by its proper food, and has only to 

 stretch roots downwards and leafy stems upwards to obtain it in 

 abundance ; for which purpose a branching form, offering a large 

 absorptive surface, is eminently suitable. Nor is a digestive cavity 

 necessary to deal with liquid and gaseous food, so such a cavity 

 is characteristically absent. Since, too, the food is in such near 

 proximity, locomotor powers would be superfluous; and equally 

 unnecessary is a well-developed nervous system to regulate loco- 

 motion and, with the aid of special sense organs, place the organism 

 in touch with the outer world. As regards any one of the higher 

 animals the case is far different. "A solution of smelling-salts in 

 water, with an infinitesimal proportion of some other saline matters, 

 contains all the elementary bodies which enter into the composition 

 of protoplasm; but, as I need hardly say, a hogshead of that fluid 

 would not keep a hungry man from starving, nor would it save 

 any animal whatever from a like fate" (Huxley). Being devoid 

 ■of the green colouring matter called chlorophyll, which enables a 

 ^reen plant to use the energy of the sun's rays for building up 

 simple food into complex substances, the animal has to live upon 

 very elaborate food, which can only be obtained from one of two 

 sources, i.e. plants or other animals. A broad popular distinction 

 has therefore been drawn between flesh-eating animals, plant- 

 eating animals, and those omnivorous forms which, like ourselves 



