228 ENTOMOLOGY 



reduction of its excess of growth or numbers, and will also set up an 

 oscillation like the preceding, except that the steps will be taken in re- 

 verse order. 



"I next point out the fact that precisely the same reasoning applies 

 to predaceous and parasitic insects. Their interests also are identical 

 with the interests of the species they parasitize or prey upon. A diminu- 

 tion of their food reacts to decrease their own numbers. They are thus 

 vitally interested in confining their depredations to the excess of indi- 

 viduals produced, or to redundant or otherwise unessential structures. 

 It is only by a sort of unlucky accident that a destructive species really 

 injures the species preyed upon. 



"The discussion has thus far affected only such organisms as are 

 confined to a single species. It remains to see how it applies to such as 

 have several sources of support open to them, such, for instance, as 

 feed indifferently upon several plants or upon a variety of animals, or 

 both. Let us take, first, the case of a predaceous beetle feeding upon a 

 variety of other insects, either indifferently, upon whatever species is 

 most numerous or most accessible, or preferably upon certain species, 

 resorting to others only in case of an insufficiency of its favorite food. 



"It is at once evident that, taking the group of its food-insects as a 

 unit, the same reasoning applies as if it were restricted to a single species 

 for food; that is, it is interested in the maintenance of these food-species 

 at the highest number consistent with the general conditions of the 

 environment, interested to confine its own depredations to that sur- 

 plus of its food which would otherwise perish if not eaten interested, 

 therefore, in establishing a rate of reproduction for itself which will not 

 unduly lessen its food supply. Its interest in the numbers of each species 

 of the group it eats will evidently be the same as its interest in the group 

 as a whole, since the group as a whole can be kept at the highest number 

 possible only by keeping each species at the highest number possible. . . . 



"This argument holds for birds as well as for insects, for animals of 

 all kinds, in fact, whether their food be mixed or simple, animal or vege- 

 table, or both. It also applies to parasitic plants. The ideal adjust- 

 ment is one in which the reproductive rate of each species should be so 

 exactly adapted to its food supply and to the various drains upon it that 

 the species preyed upon should normally produce an excess sufficient for 

 the species it supports. And this statement evidently applies through- 

 out the entire scale of being. Among all orders of plants and animals, 

 the ideal balance of Nature is one promotive of the highest good of all 

 the species. In this ideal state, towards which Nature seems continually 



