44 



Rats aiid Mice 



sufficient to control the numerical representation and distribution 

 of plants. All animals derive their sustenance from plants, either 

 directly or indirectly, or in both ways. 



Each country has its own geological and geographical history, 

 as well as its own climatic conditions ; further, climate and soil 

 vary with the district. On all these things the nature of the 

 vegetation depends ; they determine chiefly what species of plants 

 maij occur, and which of these will thrive. These things, and the 

 plant-life they control, determine further what animals may live, 

 and which of these will prosper in a given locality. All species, be 

 they animal or vegetable, are doing their utmost to feed, grow, and 

 reproduce their kind. The result of all these conditions and forces 

 is that each individual or species reacts, to a greater or lesser 

 degree, upon all other individuals or species brought directly or 

 indirectly in relation with it ; its existence and activities may 

 favour, may hinder, or may both favour and hinder the existence 

 and activities of other organisms. In this way plants and animals, 

 and the individual species of each kingdom, are everywhere so 

 closely adjusted to each other that they might be likened to the 

 cells of the honey-comb of the hive-bee ; but whereas in the honey- 

 comb we find space filled with a number of similar simple 

 geometrical forms of one magnitude, in the case of organic nature 

 we must imagine space to be filled with an enormous number of 

 figures of diverse form and size. No element can be added to or 

 taken away from such a complex without causing a more or less 

 far-reaching disturbance of the whole. There may be thousands 

 of links in the chain of cause and effect necessary to determine 

 which of two species of mouse, for example, shall predominate in 

 a given locality ; so far as human intelligence is concerned the 

 complexity of that chain is infinite. 



Every district normally contains the full number of small 

 mammals and birds it is capable of supporting ; what that number 

 is depends upon what species are present and the relative propor- 

 tions in which they occur ; these factors in turn depend upon the 

 conditions named above. But many small mammals and small 

 birds are characterized by a high fecundity ; and in each year 

 each of such species produces far greater numbers of young than 

 are necessary to maintain the normal stock; if over-population, 

 with all its attendant evils, is not to ensue, the surplus must be 

 removed — and in normal conditions it is removed. The chief 

 destructive agents are accidents, such as misadventure, sickness, 



