12 On Some Interactions of Organisms. 



the interest of a species of plant or animal and the interest 

 of its "enemies" are identical, and since the operations 

 of natural selection tend constantly to bring about an ad- 

 justment of the species and its enemies which shall best 

 promote this common interest therefore the anniMlation 

 of all the established ''^ enemies^'' of a species would., as a 

 rule., have no eifect to increase its final average numhers. 

 This being a general law, applying to all organisms, it is 

 plain that the real and final limits of a species are the 

 inorganic features of its environment — soil, climate, sea- 

 sonal peculiarities, and the like. 



In treating of the external forces brought to bear upon 

 an oscillating species to restrain its disastrous fluctuations, 

 I shall mention only a part of the organic checks to which 

 it is subject. 



It is a general truth, that those animals and plants are 

 least likely to oscillate widely which are preyed upon by 

 the greatest number of species, of the most varied habit. 

 Then the occasional diminution of a single enemy will not 

 greatly affect them, as any consequent excess of their 

 own numbers will be largely cut down by their other ene- 

 mies, and especially as, in most cases, the backward oscil- 

 lations of one set of enemies will be neutralized by the 

 forward oscillations of another set. But by the operations 

 of natural selection, most animals are compelled to main- 

 tain a varied food habit, — so that if one element fails, 

 others may be available. Thus each species preyed upon 

 is likely to have a number of enemies, which will assist 

 each other in keeping it properly in check. 



Against the uprising of inordinate numbers of insects, 

 commonly harmless but capable of becoming temporarily 

 injurious, the most valuable and reliable protection is un- 

 doubtedly afl'orded by those predaceous birds and insects 

 which eat a mixed food., so that in the absence or diminu- 

 tion of any one element of their food, their own numbers 

 are not seriously affected. Resorting, then, to other food 

 supplies, they are found ready, on occasion, for immediate 

 and overwhelming attack against any threatening foe. 

 Especially does the wonderful locomotive power of birds, 

 enabling them to escape scarcity in one region, which might 



