The vegetative body of Raffiesia may be reasonably called degenerate, but it must be asked, following MacGregor Skene, whether all the simplifications of parasitic plants deserve this term, and whether the simplifications are to be regarded as prim arily associated with the parasitism. The beginnings of reduction, e.g., in leaves and in amount of chlorophyll, are often to be seen quite apart from parasitism. Some reduction in assimilatory power, or some other inferiority in competing with rivals in crowded conditions, may prompt root-parasitism or some other parasitic dependence. And when the habit of parasitism or partial parasitism has begun, it is natural to suppose that degenerative variations, e.g., towards "golden leaves" or albinism, would be better able to survive because of the abundant nourishment sup plied by the host.
Origin of Parasitism.—(a) Many animal parasites, such as some of the nematodes, may have begun as saprophytes. Several flies that normally lay their eggs in putrefying animals, may simi larly utilize the abrased skin of one that is still living. (b) Many animals are cryptozoic, given to hiding themselves, negatively heliotropic, fond of narrow passages, and this is another way in which parasitism may arise. The commonplace is often over looked that the host is not to the parasite another organism, but merely a convenient and attractive environment. (c) But it is likely that parasitism often arose when the struggle for existence was extremely keen, when even slightly open doors were welcome. As we have indicated, the most characteristic feature in parasitism is probably a kind of constitution more inclined to drift, rather than to swim. Just as some animals became cavernicolous, others became parasitic. (d) Crustacean parasites are particularly inter esting because they afford many illustrations of parasitism re stricted to the females. The parasitic habit arose in connection with the advantage of securing sheltered nooks for liberating the eggs or offspring. (e) Where the sex dimorphism is very pro nounced, as in Bonellia, many copepods, and two mid-water angler fish, there would be an advantage to the males and to the species, perhaps even to the females, in the occurrence of parasitic males, securing fertilization. In the anglers alluded to, which occur sparsely in thinly-peopled waters, the female carries the minute male, and all his nourishment is derived from an organic connec tion between his head and the blood-vessels of some part of her skin. (f) It is possible that parasitism has in some cases evolved from shelter-associations and from commensalism (see SYM BIOSIS) ; as we have indicated it is often difficult to draw the line. (g) Probably one parasitism has frequently arisen from another. That is, the parasite of a freshwater crustacean may become adapted to become secondarily parasitic in a freshwater fish which habitually feeds on the first host. An elaboration of this
extension of range comes about when different phases of life are restricted to each df the two hosts, a common punctuation being that an asexual phase occurs in the one host and a sexual phase in the other. Leuckart, who discussed the problem carefully, came to the conclusion that the "intermediate host," now containing the young non-reproductive phase, was the original host, and that the "secondary or definitive host" has been added on. Yet it is conceivable that in some cases the intermediate hosts of the im mature stages have been intercalated. When there are two hosts, there is usually, though not invariably, this relation between them, that the intermediate host is part of the normal diet of the definitive host.
Damage Done to Host.—It is usual to emphasize the fact that it is usually not in the parasite's biological interest to destroy its host. In many cases, e.g., follicle mites in man, or Monocystis in earthworms, the parasite is small compared with its host and the damage done may be unimportant. Yet a heavy infection with threadworms may be fatal to a horse, and if ichneumon-grubs are regarded as parasites, an interpretation here departed from, they are obviously fatal to their caterpillar hosts. Much depends on the numerical strength of the infection; a few gall-larvae, each im prisoned in its gall, may be regarded as trivial, but a multiple infection of a currant bush with "big bud" mites may be fatal. Then a distinction must be drawn between parasites that multiply in their host, as nematode worms often do, and those, like tape worms, that cannot increase in number within the same animal. It may be that rapidly destructive parasites have been persistently eliminated in the course of evolution, as would naturally happen if they destroyed their host before becoming themselves reproduc tive. But the large fact to be emphasized is that in many cases a give-and-take relation is established between the parasite and the host, such that the parasite does not get the upper hand and the host is not too seriously prejudiced. In the intestinal caeca of the grouse there are often thousands of invisible transparent nema todes ( Trichostrongylus pergracilis), whose early stages are found on the heather. If the grouse is otherwise in good condition, its nematodes seem to be unimportant, but if the grouse be constitu tionally below par, the parasites may multiply excessively (i o,000 in one bird) and fatally. When parasites or quasi-parasites prove quickly destructive, it is usually when they find their way into a new host that has no natural counteractives to their influence. This is illustrated by bacteria when they find themselves in a new host which has not the wonted natural checks, such as anti-bodies.