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that the average number of each species is maintained from 

 year to year, that there may be a shght fluctuation in abund- 

 ance comparing one year with its predecessor or successor ; 

 but we know that on the whole the single pair of 1880 was 

 represented by a single pair in i8go, and that the single pair 

 of i8go will be represented by a single pair in igoo. We 

 become aware after consideration of facts like these that a 

 vast amount of destruction must take place year by year, 

 and that only an exceedingly small percentage of the eggs 

 laid can ever become perfect insects. 



What causes this destruction ? The answer is necessarily 

 complex ; but however complex it may be, it brings us to 

 the third important factor in the Darwinian theory of 

 evolution, the tremendous struggle for existence that exists 

 amongst individuals of the same species, and the conse- 

 quent survival of the fittest. The destruction may be 

 brought about by a variety of causes : (i) Atmospheric 

 conditions — rain, floods, storms, cold, drought, excessive 

 heat or cold, &c. (2) Natural enemies — dependent, of 

 course, entirely on the class or species of animal or plant. 

 (3) Changes brought about by man — agricultural improve- 

 ments, drainage, &c. (4) Starvation — owing to destruction 

 of food-plant, either from natural causes or from the abund- 

 ance of individuals devouring the particular food-plant, 

 often converted here into a struggle with individuals of its 

 own kind. (5) Disease. There are, of course, many other 

 factors which will occur as readily to any field naturalist as 

 to myself. 



Probably the opponents of the theory of natural selection 

 have objected most persistently and strongly to the small 

 amount of variation noticeable amongst the individuals of 

 most species from a given district. This is, of course, true 

 up to a point, necessarily true ; but the objectors forget that 

 these individuals which have reached maturity are already 

 the selected individuals of their race, having been already 

 selected, if insects, in the oval, larval, pupal, and imaginal 

 stages, for some qualities possessed which were lacking 

 in and led to the destruction of their companions. The 

 comparison is being made between those individuals which 

 have been preferred owing to their having fallen in all their 

 stages within the limits of the average type of the species, 

 and because they responded best to the necessities of their 

 environment in all their different stages. Surely this is 

 sufficient to explain why the individuals appear to exhibit 

 so small an amount of variation, especially when collected 



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