The life of a parasite, contrary to general opinion, is not an easy one, it is full of dangers and the parasite is rigorously circumscribed. Parasitism is an achievement and the term degenerate is not aptly applied to this mode of living. Parasites are adapted to this mode of life in two general respects, namely (1) physiologically and (2) morphologically , and these adaptations become the more intense and exclusive as this mode of life progresses A true parasite should not destroy its host,—the life of the host and that of the parasite have become closely tuned to each other and the slightest change in the mode of life of either may result in serious disturbances in the life of the other. It appears that many of the more dangerous blood sucking species of insects have a rather benign “bite,” note the difference in the bite of Anopheles maculipennis , an important malaria vector, and Aedes dorsalis , a salt marsh species. A blood sucking insect that is to become a successful carrier of human disease will probably have to make its blood-lust less offensive to its host. The laws governing the development of a successful parasite are operative in the development of all successful living beings, it is therefore necessary to study the effects of parasitism on the host and on the parasite from a more comprehensive biological point of view.