6 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



necessity for powers of locomotion, which are unnecessary to 

 higher plants. A compact form is obviously advantageous under 

 the circumstances, and the possession of a well-developed nervous 

 system and sense-organs bringing the various activities of the 

 animal into touch with one another and the outer world becomes 

 readily intelligible. 



Plants and animals, however, must be regarded as of common 

 descent, forming as it were the two diverging limbs of a V, the 

 higher forms being situated at the ends of those limbs, and 

 the lower forms near the point of union. Hence, as we pass 

 down in the scale the differences between the two kingdoms 

 become less and less marked, until, in the simplest cases, it is 

 often scarcely possible to say whether a given organism be plant 

 or animal. 



The distinction which perhaps holds most generally is found in 

 the nature of the food, and plants are also usually characterized 

 by the presence of protective membranes composed of cellulose 

 (C 5 H 10 5 )n, a substance closely allied to starch. It is, indeed, 

 the presence of these membranes which renders it necessary that 

 the food should be in a gaseous or liquid condition. 



3. Distinctions between Living and Non-Living Matter. An 

 organism is bounded by curved surfaces, while masses of non- 

 living matter are either shapeless (amorphous) or else of crystalline 

 form, in which case they are usually limited by flat faces meeting 

 in straight edges. An organism, too, is of excessively compli- 

 cated physical and chemical structure, and its living part is 

 always composed of a substance known as protoplasm, of which 

 more will be said in the sequel. This complexity is related to 

 a process of constant chemical change (metabolism), involving 

 continual loss of substance, which must be compensated by the 

 intaking of food. This is built up into fresh protoplasmic mole- 

 cules, which are intercalated between those already existing. By 

 this process of intussusception growth may be effected, up to a 

 certain limit in the case of each organism. A mineral mass, say, 

 for example, a crystal of copper sulphate, is not of such exceed- 

 ingly complex nature, nor are its molecules subject to constant 

 down-breaking and up-building. It may be kept in an unaltered 

 form for an indefinite period, and since it does not eliminate 

 waste products does not require food. If placed in a saturated 

 solution of copper sulphate it exhibits a kind of growth, by 



