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.
- Factor No. 7, taken alone or combined with the preceding factor, has a prime importance in preserving the host species from destruction by the parasitic species. Three examples will serve to illustrate this:
First Example.—Encyrtus (Ageniaspis) fuscicollis is a hymenopterous parasite whose power of prolification is immense, since, as I have shown in an earlier memoir, it presents the very exceptional phenomenon of polyembryony, that is to say, that a single one of its eggs can give birth to more than 100 individuals, capable of multiplying in this way.
Now this Encyrtus lays its eggs in the eggs of a moth of the genus Hyponomeuta, which has only a single generation each year, as has the Encyrtus itself. Under these conditions it may be asked how the Hyponomeutas, instead of being promptly annihilated, are, on the contrary, capable of multiplying in certain years to the point of being destructive to fruit trees during their larval stage, and this in spite of a number of other parasites, particularly Tachinids. The reasons are certainly numerous; but the one to which we wish to call attention is, that the time of the swarming of the Encyrtus is notably shorter than the egg-laying period of the Hyponomeutas. However immense may be the number of Encyrtids that appear in a season, one may be certain that all of