THE DARWINIAN THEORY 49 



The average number of individuals in a species depends on many- 

 other conditions, especially on the extent of the available area, and 

 on the amount of the food-supply in relation to the size of body 

 in the species. I cannot dwell on this now, but I wish to point out 

 that, for the continuance of a species, it is indifferent whether it is 

 ' frequent ' or ' rare,' if we presuppose that its normal number remains 

 on an average constant for centuries, that is, that its fertility suffices 

 to make good the continual losses through enemies and other causes 

 of elimination. One would be inclined to conclude from such eases 

 of sudden and enormous increase in the number of individuals as 

 these caterpillar-blights, that enemies and other causes of destruction 

 played the major part in the regulation of the normal number of the 

 species. But this is only apparently the case. Enemies necessitate 

 a certain fertility in the species on which they prey, so that the 

 elimination in each generation may be made good ; but the number of 

 pairs capable of reproduction is not thereby decisively determined. 

 We must not forget that the numb>er of enemies is also, on the other 

 hand, dependent on the number of victims, and that the normal 

 number of enemies must rise and fall with that of the species preyed 

 upon. 



For this reason, such an enormous increase as that of the cater- 

 pillars cannot last long; it carries its corrective in itself. The 

 appearance of the caterpillars in such enormous numbers in itself 

 increases the host of their enemies; singing-birds, ichneumon-flies, 

 beetle-grubs, and predaceous beetles find abundant and available 

 food, and therefore reproduce and multiply so rapidly, that, with the 

 help of the caterpillar's plant-enemies, especially the insect-destroying 

 fungi, they soon reduce the caterpillars to their normal number, or 

 even below it. But then the reverse process begins ; the enemies of 

 the caterpillars diminish because their food has become scarce, and 

 their normal number is lowered, while that of the caterpillars 

 gradually rises again. 



When the number of foxes in a hunting district increases, the 

 number of the hares that they prey upon diminishes, and, on the other 1 . 



hand, the decimating of the foxes by Man brings about an increase vnyc/^ 

 the number of hares in the district. Under natural conditions, that \ 

 is, without the intervention of Man, there would be a constant y^ 

 balancing of the numbers of hares and foxes, for every noteworthy 

 increase of the hares would be followed by a similar increase of foxes, 

 and this, in its turn, would diminish the number of hares, so that 

 they would no longer suffice for the support of so many foxes, and 

 these would decrease ih number again, until the number of hares had 



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