356 POPULAR SCIENCE MONTHLY 



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 theoretic- 

 ally 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 ex- 

 clusive 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 para- 

 sites. 



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. 4 



4 Factor No. 7, taken alone or combined with the preceding factor, has a 

 prime importance in preserving the host species from destruction by the para- 

 sitic 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 Hypo- 

 nomeuta, 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 atten- 

 tion 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 



