6 AN INTRODUCTION TO ZOOLOGY 



that we term the power of nutrition, and it always involves certain 

 subsidiary processes. Just as a fire needs a constant supply of 

 fuel, so an organism must have food to maintain its vitality. Plants 

 in general grow in such situations that they can obtain their food 

 from the water, the soil and the air surrounding them, and so are 

 fixed, whereas animals usually have to seek out their food, and are, 

 in consequence, able to move from place to place. The food, when 

 it reaches the organism, is taken in or ingested in some way or 

 other, and this is the first process of nutrition. Many of the sub- 

 stances ingested can be dealt with straight away, because they are 

 soluble, but a number of others are insoluble or unable to pass 

 into the living substance until they are altered in some way or 

 other, usually, in the case of animals, reduced to more simple 

 compounds. This, then, is the second process of nutrition, namely, 

 digestion, or the changing of the chemical nature of the food in 

 such a way that it can be taken up into the organism. The third 

 step consists of the building up of these substances, chemically 

 speaking, relatively simple, into the complex compounds character- 

 istic of living beings, and this is termed assimilation. Lastly, in 

 order to obtain the substances required as food, it is nearly always 

 necessary to take in certain other materials that are not wanted, 

 and often, too, materials that cannot be rendered soluble by the 

 process of digestion. This is noticeably the case in animals, and 

 we find this waste, or better, not-utilisable matter, voided from the 

 body as faeces, a process termed egestion. 



This is, as it were, the " receipts " side of the account of living 

 matter, and it results in the replacement of used-up material, and 

 perhaps an increase in size or growth or a storage of certain reserve 

 food substances until such time as they may be required. To one 

 part of this, namely, the building up of the living matter or Anabo- 

 lism, we shall return later when we have considered this substance 

 from the physical and chemical points of view. 



On the other hand, we have also to consider the " expendi- 

 ture " side of the account. In order that these complicated chemical 

 changes may go on, it is necessary for the organism to obtain 

 Oxygen, and this it gets, as a rule, from the air or water surrounding 

 it by a process termed Respiration. Again, just as a fire requires 

 Oxygen to continue burning, so also oxygen is a necessity to living 

 matter, and a further parallel between the two can also be drawn. 

 The burning of the fire results in the formation of various waste 

 matters, taking the form of gases and ash. The activities of an 

 organism also produce waste products. One of these is the gas 

 Carbon Dioxide, and quite frequently the same mechanism that 

 serves for obtaining the oxygen supply is also utilised to get rid of 



