HIGH SCHOOL ZOOLOGY. 263 



4. They may be stated as follows : — (1) Life is always asso- 

 ciated with protoplasm, which has accoi-dingly been termed the 

 " physical basis of life " Even in its simj)lest forms, this sub- 

 stance is undoubtedly extremely complex from a chemical stand- 

 point, while in its more differentiated forms, it contains tempor- 

 arily within it innumerable " organic compounds " which form 

 a consideraVjle part of the subject matter of Organic Chemistry. 

 All forms of })rotoplasm are constituted largely of " proteids " 

 — complex compounds of Carbon, Hydrogen, Oxygen and Nitro- 

 gen, with small proportions of Phosphorus and Sulphur, but 

 they also form within them Carbon compounds of greater sim- 

 plicity, such as starch, sugar, fat, etc., which, however, do not 

 occur in nature except as the products of the organic world. 

 (2) The life of protoplasm (as manifested, for example, by irrita- 

 bility and contractility) is accompanied by its partial destruction, 

 and this involves repair by the incorporation and assimilation 

 of new matter, or, in other words, the ingestion of food and 

 changes whereby this food can be rendered available for replace- 

 ment of the material destroyed. (3) If the income of food is 

 in excess of the expenditure by waste, growth results; but each 

 individual organism has a limit of size which it cannot exceed. 

 A process succeeding the attainment of the full size in the 

 simplest organisms is that of division, whereupon the life of the 

 individual terminates and a new generation composed of two 

 new individuals replaces it. Similar phenomena of death and 

 reproduction occur also in the higher organisms. Nothing like 

 these charactei'S is to be met with in the Inorganic world, but this 

 is not the only reason for studying the life of plants and animals 

 together, for, apart from the circumstance that there are cer- 

 tain ox'ganisms (IX, 1 9) which can hardly be said to have struck 

 out on either of the diverging paths which lead to plant 

 and animal peculiai'ities, there is so much interdependence 

 between the two, that a knowledge of the life of either is not 

 complete without considering both, and indeed this interdepen- 



