There are numerous other flies and insects that serve as carriers, and there are numerous diseases due to more or less uncleanly habits, that are carried by insects; but the process is so simple and direct that it offers little of scientific interest.
Much more complicated is that transmission of disease in which the insects act as intermediate hosts, and here again the members of the order Diptera lead the way. We assume that man is highest in the scale of vertebrate development, and it is no part of my thesis to dispute this. It is equally assumed that the Diptera are most highly specialized in their development among the insects, and it is significant that the association of humanity with these flies has been so close and
so long continued that it has been possible for a common parasite to develop whose continuance depends absolutely upon the intimate association of the two.
If all insects were eliminated absolutely, typhoid, cholera and associated diseases might still continue to plague mankind; but eliminate Stegomyia calopus and yellow fever would be equally eliminated, because without that particular species the parasite causing the disease would find it impossible to continue its existence. Not that we know very much about the "germ" of yellow fever, and its life-cycle is largely guess-work; but we do know enough, from direct observation and experiment, to warrant the statements that I have just made.
It is different in the case of malerial fevers and their association with certain species of Anopheles. Here we do know from direct
