34 WHAT IS LIFE 



it is quite clear that the various excreta are got rid 

 of because they are of no further use to the animal 

 excreting them, yet they may be of great service 

 for purposes of fertilisation. 



Finally, as regards this question of nutrition and 

 digestion, the result of this taking in of food, and 

 of the changes which take place in it, is an increase 

 in size always, however, within certain limits 

 peculiar to each species, very wide limits in some 

 cases but still definite limits, so that one can always 

 say of any given species that it will not exceed a 

 certain size or fall below a given limit. 



Respiration Eespiration is a process closely associated with 

 those which we have been considering. We cannot 

 see it taking place in the amoeba but we must 

 suppose that the interchange of gases which con- 

 stitutes this process takes place all over the surface 

 of the creature. 



But if we cannot see the respiration taking 

 place there is one further very remarkable pheno- 

 menon which we can study and that is the process 



Reproduc- of reproduction. In the amoeba this consists of a 

 division, first of the nucleus and secondly of the 

 whole cell into two portions, so that where there 

 was previously but one amoeba, now there are two. 

 It is obvious that in such a case it is impossible to 

 speak of mother and daughter, since both cells 

 might lay claim to either title, each representing 



