Leuckart therefore regards the change of hosts as secondary, so does Sabatier. The latter, however, adduces other reasons for this (lack of clinging organs and the necessity to develop them in an intermediary stage); but in this connection he only considers the Cestoda. In opposition to Leuckart, R. Moniez, however, is convinced that the migrations of the helminthes, as well as the system of intermediate hosts, represent the original order of things. Moniez traces all Entozoa from saprophytes, but only a few of these were able to settle directly in the intestine and there continue their development. These are forms that at the present day still lack an intermediate host, such as _Trichocephalus_, _Ascaris_, and _Oxyuris_. In most other cases the embryos, however, consisted of such saprophytes as were, in other respects, suitable to become parasites, but were incapable of resisting the mechanical and chemical influences of the intestinal contents. They were therefore obliged to leave the intestine at once, and accomplished this by penetrating the intestinal walls and burrowing in the tissues of their carriers. In this position, assisted by the favourable conditions of nutrition, they could attain a relatively high degree of development. Mechanical reasons prevented a return to the intestines, where the eggs could be deposited. Most of them doubtless died off as parasites, as also their young stages do at present when they penetrate wrong hosts. Some of them, nevertheless, passively reached the intestine of beasts of prey. Many were destroyed in the process of mastication; for a small part, however, there was the chance of reaching the intestine of a beast of prey undamaged, and there, having become larger and more capable of resistance, maturity was attained. By means of this incidental coincidence of various favourable circumstances, these processes, according to Moniez, have been established by heredity and have become normal.
This is not the place to express an opinion either for or against the various hypotheses advanced, but the existence of these diametrically opposed views alone will show the great difficulty of the question. Independently, however, it appears more natural to come to the conclusion that parasitism, as well as change of hosts, were gradual transitions.