Page:Popular Science Monthly Volume 79.djvu/438

This page has been proofread, but needs to be validated.

posing animals or plants or from the excretions of the former. The mutualist, finally, as the name implies, lives in a condition of balanced energetic or nutritional cooperation with another organism.

Of all these types of reactions to the food supply, parasitism is far and away the most prevalent; so prevalent, in fact, that it may be doubted whether there is any animal that does not resort to it, at least during a brief portion of its life, even if this be only during the period when, as an egg, it is drawing its supply of food-yolk from its parent. That parasitism has been most frequently developed from predatism is certain, that it may occasionally have its origin in commensalism, mutualism or scavengerism is highly probable, that it can, especially when it affects a considerable portion of the life-cycle of an organism, develop into anything but a more extreme form of parasitism, is very doubtful.

It would be easy to show by the citation of many examples that parasitism is an extremely protean phenomenon, one which escapes through the meshes of any net of scholastic definitions in which we may endeavor to confine it. Nor is this surprising when we stop to consider its great prevalence and the fact that during the course of time the organic world, pari passu with its increasing differentiation, has become ever more and more heavily weighted with parasitism and mutualism. That this nutritive dependence of organisms on one another has been steadily growing during paleontological time is clearly seen in the comparatively recent development of viviparity in mammals and many other animals, in the development of the alternating generations of plants into a condition in which the gametophyte is parasitic on the sporophyte (gymnosperms and angiosperms) or the sporophyte on the gametophyte (ferns and mosses), in the increasing mutualistic relations between insects and angiosperms, in the enormous development of parasitism among the highest orders of insects, the Diptera, Hymenoptera, Coleoptera, Lepidoptera and Homoptera, which are not known to have existed before Jurassic and Triassic times, and even in many apparently more primitive parasites like the true lice, bird lice, bat lice, fleas and many tape worms, flukes and round worms, which could not have developed till after their mammalian and avian hosts had made their appearance. Social life, too, which is hardly more than a mixture of parasitism and mutualism, shows a similarly recent development. Man himself, with whom we do not commonly associate the idea of parasitism, although the term is derived from a certain type of man well known to the ancient Greeks, not infrequently displays an extraordinary variety of parasitic activities. As an embryo he is always entoparasitic, using his allantois in a manner that vividly suggests the rootsystem of a Sacculina attached to a crab. At birth he becomes a kind of ectoparasite on his mother or nurse, and throughout his childhood and youth he is commonly what might be called a family parasite, de-