Page:Popular Science Monthly Volume 79.djvu/437

Rh Boston. Here for several years past great numbers of parasites have been received from Europe and northern Asia, carefully reared and studied, and, when found to be sufficiently promising, liberated in the hope that they will multiply and eventually control the gypsy and brown-tail moths.

The fact that such economic uses have been suggested for insect and not for any other parasites seems to imply that the former must be peculiar in certain important particulars. This I believe to be true and has led to the following considerations. That I have chosen to read them to you, who are primarily interested in the problems of zoology in its broadest sense, is due to a conviction on my part that many of the accounts of parasitism, even in the best of our zoological hand-books, are more or less one-sided and anthropomorphic, probably as a result of the stepmotherly treatment necessarily bestowed upon the insects in such treatises. Before I say more about the insects, however, I wish to make a few remarks on animal parasitism in general.

Parasitism is, of course, a form of "behavior," and may be described as one of several complex types of the reactions of organisms to the most important source of their energy, their food supply. Other reactions to this element of the environment are predatism, commensalism, scavengerism and mutualism. There is in the main sufficient consensus of opinion concerning the distinctions between these different phenomena. Predatory animals kill other animals and devour them wholly or in part. Parasites put other organisms in the position of "hosts" by living directly on their tissues in such a manner as not to cause their immediate death. The parasite thus draws indirectly on the food supply of another organism by permitting or compelling it to do the hard work of procuring the food and of converting it into much more accessible and much more easily assimilable compounds. The parasite may be said, therefore, to use its host as an instrument not only for procuring, but for predigesting, its food. The commensal also uses another animal as an instrument, but merely in gaining access to a food-supply which the latter has procured but has not yet assimilated. The scavenger, like the saprophyte among plants, may be described as a parasite of the dead, deriving its sustenance from