Page:Popular Science Monthly Volume 72.djvu/360

 The conditions and factors which control the relations of beings among themselves are so numerous and so complex that to interpret them and render them intelligible one is forced to speak more or less theoretically in considering only certain of the causes and in momentarily omitting the others.

In fact, the regulation can in no case be considered as the exclusive result of the action of any given parasitic species, of which the fecundity will be proportioned to that of the host species and in such relation that it maintains a constant numerical tax.

The fecundity of the parasitic species is only one of the factors which determines this equilibrium. If it is true that it is of prime importance, that fact should not prevent us from taking account of the others. There are a number of these, as follows:

First.—The hyperparasites, or secondary parasites, living at the expense of the primary parasite, and having themselves tertiary parasites.

Second.—The coparasites, that is to say, other species living in the same host.

Third.—Other plant-feeding species occurring with the host species.

Fourth.—The enemies of all insects (insectivorous birds, etc.), attacking both the plant-feeding species and the parasitic species.

Fifth.—Climatic conditions influencing in a favorable or in an unfavorable way either the host species or the parasites, the hyper-parasites, or the enemies of all kinds liable to attack the insect.

Sixth.—The rapidity with which the generations are developed,—of the host species, on the one side, and the parasitic on the other.

Seventh.—The tendency in the plant-feeding species to retard the development of certain individuals of a given generation for a longer or shorter time.