434 THE POPULAR SCIENCE MONTHLY 



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 genera- 

 tions 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, Hymen- 

 optera, Coleoptera, Lepidoptera and Homoptera, which are not known to 

 have existed before Jurassic and Triassic times, and even in many ap- 

 parently 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 para- 

 sitism, although the term is derived from a certain type of man well 

 known to the ancient Greeks, not infrequently displays an extraordi- 

 nary variety of parasitic activities. As an embryo he is always ento- 

 parasitic, using his allantois in a manner that vividly suggests the root- 

 system 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- 



