BALANCE OF FOOD AND LOSS. 41 



tliere is a second, which in later life is at least equally press- 

 ing. If tliere were no otter cause, the animal might cease to 

 eat as soon as it had attained its full growth. But everyone 

 knows that regular and, in some cases, numerous meals are 

 required, and consequently every animal is forced, to the -very 

 last day of its existence, to seek food, although growth has long 

 since ceased. The reason for this is very simple. That sum of 

 functional activity which we call life can only be maintained by 

 using up the organic matter contained in the tissues of the 

 living body. The activity of the muscles and of the brain, the 

 sensitiveness of the sense-organs to external impressions, the 

 secretion of urine or perspiration, respiration, propagation, and 

 the assimilation of food — in short, all the vital processes that are 

 carried on in the living individual — are only possible through the 

 consumption, or, more correctly, the decomposition, of a corre- 

 sponding amount of the organic matters contained in the organs 

 that are exercised. The minimum of matter thus destroyed 

 may be greater or less in different animals, sometimes even 

 inappreciably small; but the loss of even this minimum of 

 organic matter must sooner or later endanger the life of the 

 animal if it is not soon made good. In order to make it good 

 and to be at the same time in a position to carry on uninter- 

 ruptedly the normal process of loss of its own tissues by secre- 

 tion, the animal must consume nourishment in various pro- 

 portions according to its needs. There are apparent exceptions 

 to this rule : for instance, the well-known cases of animals — 

 Amphibia, Mollusca, and others — which are able to live for years 

 without food. I myself kept various species of land-snails for 

 years wrapped in paper and quite dry in wooden boxes, and thus 

 wholly without food, and many of them are at this day alive 

 and active.^ The explanation of this striking instance is easily 

 found. The amount of nourishment required daily by any 

 animal must naturally be equivalent to the organic matter 

 which is daily used up in the various organs to keep up the 

 vital processes : the more active an animal is, the more food will 

 it require. But the vital processes of animals that are as low in 

 the scale as the Amphibia or Univalves are extremely feeble ; 

 their respiration, even under the agitating influence of pro- 



