like every other healthy organism, can afford to sacrifice to the accidents of its environment.[1]
The parasite not only tends to restrict itself to the use of a particular host as a food-procuring instrument, but is also compelled to exercise the most exquisite care in the use of this instrument. From the very nature of the situation, therefore, parasitism is an extremely precarious type of behavior. But this is true also of all highly specialized behavior, that of biologists included, and points the way to, but does not constitute, the real difficulty with parasitism. This, I take it, is the suppression of the voluntary movements, which necessarily results from the intimate host relations, especially when these are confined, as is so often the case, to some one particular organ or tissue. It is not, therefore, the parasite's habit of taking something for nothing from another organism, that is so fatal, for all creatures, in matters relating to nutrition, find it more blessed to receive than to give, but the acceptance of the most important supply of its energy under conditions that preclude an exercise of the muscular and hence also of the sensory and nervous activities and restrict its vital activities to a round of assimilation, metabolism and reproduction. This unbalancing of functions is probably hastened by a kind of intraorganismal parasitism or "Kampf der Theile" in Roux's sense, the alimentary and reproductive tissues drawing their nutriment not only from the host but also from the more inactive tissues of the parasite's own body. That this torpor, or inactivity of the neuromuscular system is at the bottom of the peculiar disability of parasites is shown by many non-parasitic organisms, which have easy access to an abundant food supply consisting of dead or inorganic substances. Most plants and many invertebrates, such as the barnacles, and especially the scavengers among insects, exhibit essentially the same modifications as parasites. In fact, the larval stages of many insects that feed on carrion or decomposing animal and vegetable matter, are quite indistinguishable from parasitic larvæ. This and the further fact that plant-eating species are not generally regarded as parasites by entomologists have led to considerable confusion in certain accounts of insect parasitism.
While most parasites among the lower invertebrates have never succeeded in freeing themselves from the tyranny of the host relation and the fatal torpor to which it inevitably leads, this is, as we have seen, by no means true of the typical insect parasites. To the ontogeny of these organisms the dictum "once a parasite, always a parasite" most cer-
- ↑ Within this "margin of vitality" must also be included the reproductivity of the host species. Thus certain ants, like Formica fusca, throughout the north temperate zone, are able to survive the inroads of a number of parasitic ants (Polyergus rufescens, Formica sanguinea, F. rufa, F. exsecta and many allied species), largely on account of its great reproductive powers, coupled with an ability to live in the most diverse physiographic conditions.
