56 BASIS OF THE PKOBLEM 



Fecundity is, therefore, in some manner connected with this 

 fact that the great majority of fertilized eggs do not give rise to 

 adults, and in order to throw further light upon this connexion 

 we must ask how it is that the young perish. 



18. To make clear how it is that the young of every species 

 perish on so large a scale, it is necessary to refer to the inter- 

 dependence of all living organisms. This can perhaps best be 

 illustrated by reference to the chief distinction between animals 

 and plants. A difference in the mode of nutrition is that which 

 chiefly distinguishes animals from plants. There are other 

 differences but their importance is small compared with that we 

 may now describe. The need of food is common to all living things 

 and is due to the nature of the living substance called protoplasm 

 the physical basis of all life. Protoplasm is of a very complex 

 constitution. It is for ever wasting away and if life is to be 

 preserved food must be supplied to compensate for the loss. 

 The need is as great among plants as it is among animals, but the 

 means of supplying it are fundamentally different. 



Plants feed upon very simple substances salts of nitric acid, 

 salts of ammonia, and carbonic acid. Among green plants 

 carbonic acid is taken in from the air through small apertures in 

 the leaves known as stomata ; the salts of nitric acid and ammonia 

 are absorbed from solution in the water of the soil. Carbonic 

 acid and water are synthesized within the cells of the plant into 

 starch ; starch is converted into sugar and the sugar is combined 

 with the salts of nitric acid and ammonia to form amino-acids 

 which are eventually transformed into proteids. Thus the plant 

 makes good the unavoidable waste of its living substance by 

 elaborating the highly complex protoplasm from the simplest 

 elements. 



The method pursued by animals is entirely different. They 

 feed upon complicated substances, which may be divided into 

 proteids, fats, and carbo-hydrates. These substances do not take 

 their places directly in the living cells of the body ; they first 

 undergo a process of digestion, after which they are assimilated. 

 Digestion involves the breaking down of these complex foods to 

 a certain stage ; proteids, for example, are reduced to amino- 

 acids, and starches to sugars. In these forms they are soluble 

 and are taken up by the walls of the alimentary canal and are 

 afterwards resynthesized into proteids and starches. 



