16 GENERAL CONCEPTS 



The increase in mass may be brought about by an increase in the size 

 of the individual cells, or by an increase in the number ot cells. An 

 increase in cell size may occur by the simple uptake oi water, but this is 

 not generally considered to be growth. The term, growth, is restricted to 

 those processes which increase the amount of living substance of the 

 body. This is commonly measured by the amount of nitrogen, of protein 

 or of nucleic acid (see p. 29) present, but objections may be raised to 

 the use of any single one of these parameters. Growth may be uniform 

 in the several parts of an organism, or, perhaps more commonly, growth 

 is differential, greater in some parts than in others, so that the body 

 proportions change as growth occurs. 



Growth may occur throughout the life span of an organism or may 

 be restricted to a part of it. One of the truly remarkable aspects of the 

 process is that each organ continues to function while undergoing 

 growth. 



Reproduction. Yet another characteristic of living things is their 

 ability to reproduce their kind. Since individual animals grow old and 

 die, the survival of the species depends upon the replacmg of these indi- 

 viduals by new ones. Although at one time worms were believed to arise 

 from horse hairs in a trough of water, maggots from decaying meat and 

 frogs from the mud of the Nile, we now know that each can come only 

 from previously existing ones. One of tlie fundamental tenets of biology 

 is that "all life comes only from living things." The process of repro- 

 duction may be as simple as the splitting ot one individual into two. 

 In most animals, however, it involves the production of specialized eggs 

 and sperm which unite to form the zygote or fertilized egg, from which 

 the new organism develops. In some animals, the liver flukes for ex- 

 ample, reproduction involves several quite different forms, each of which 

 gives rise to the next in succession until the cycle is completed and the 

 adult reappears. 



Adaptation. To survive, an animal or plant must be adapted to 

 its surroundings. Each particular species can achieve adaptation either 

 by seeking out a suitable environment or by undergoing modifications 

 to make it more fitted to its present surroundings. This ability to adapt 

 is a further characteristic of all living things. Adaptation may involve 

 immediate changes which depend upon the irritability of protoplasm, 

 or it may be the result of a long-term process of mutation and selection 

 (p. 704). It is obvious that no single kind of organism can adapt to all 

 the conceivable kinds of environment, hence there will be certain areas 

 where it cannot survive. The list of factors which may limit the distribu- 

 tion of a species is almost endless: water, light, temperature, food, preda- 

 tors, other organisms, and so on. 



6. Protoplasm 



The living substance that makes up each cell is known as proto- 

 plasm. We cannot see directly the protoplasm of most animals, for it is 

 hidden by a protective covering of skin, hair or shell. In an animal such 

 as the ameba, however, we can observe naked protoplasm and find that 



