CHAPTER VII 

 GROWTH AND REPRODUCTION 



IT is a somewhat remarkable fact that by far the larger number of 

 the different species of living organisms have a certain size to 

 which they grow, and individuals deviate but little from this size. 

 One cannot make any general statement as to why this is so, but 

 it seems likely that the causes are various, sometimes mechanical, 

 sometimes, perhaps, due to the digestive arrangements being 

 unable to supply a larger bulk with the necessary food. If a grow- 

 ing rat is supplied with a diet which is adequate to maintain it at 

 a small size, but inadequate for growth, it may remain at this size 

 long after it ought to have been fully grown. If, then, it be given 

 a complete diet, it grows to the normal size, but not beyond it. 



Similarly, it is not easy to say why an organism should, sooner 

 or later, cease to perform those functions which we call "life," die 

 and disintegrate. It is to be presumed that some essential part of 

 the cell machinery cannot be replaced when it has worn out, 

 although this conception does not lead us far. 



In any case, the fact of the death of the individual makes it 

 necessary that provision be made for the continuance of the race in 

 new individuals for the production of a young and vigorous new 

 generation by what indeed we know as " reproduction'' 



In such lowly organisms as the bacteria, we find that, when an 

 individual has grown to a certain size, it simply divides into two, 

 and the process goes on at a great rate under favourable conditions. 

 When supply of food is limited, or the medium in which they are 

 growing dries up, a part or the whole of each individual organism 

 collects into a mass and becomes surrounded by a layer of resistant 

 material, apparently almost impervious to water. These are called 

 "spores." In this condition, bacteria are much more difficult to kill 

 by heat. They remain dormant, but become active forms again in 

 the presence of water and food. It is not, however, all kinds of 

 bacteria that form spores. 



The nucleated unicellular organisms, including most of the 

 protozoa and algae, also multiply for the most part by simple 

 division, but a new phenomenon makes its appearance here, as we 



