Page:Popular Science Monthly Volume 75.djvu/358

354 fossil forms to living forms and he has succeeded in tracing the remote ancestry of a number of living races of mammals.

The Constancy of Nature.—As one great result of the investigations of the nature seekers, there was established a belief in the constancy of nature, and from the work of the zoologists in particular came the idea that all animal life is the result of one orderly progress. Animal organization leads up to the structure of the human body, and on this account there has always been a tender point in discussing the evidences as to man's place in nature.

This belief in the constancy of nature was a great step in intellectual development. In its broad application it means that the entire universe and all on it is the result of an orderly and well-directed progress. It leaves no room for the idea of chance. Remote ancestral man did not rise by chance from the animal series. The gill-clefts in the human embryo are not there by chance. Their presence has some significance, if haply we may find it. The great service of establishing the idea of orderly progress in nature is part of the heritage of work already done. The idea, in so far as it involves living and fossil forms of animals, is owing to the progress of zoology.

Some Practical Applications.—Let us now consider secondly some of the applications of zoological advances to the benefit of mankind. It was owing to the cooperation of botany and zoology that the germ theory of disease was established. The bacteria are, of course, plants. The method of studying their action on animals is zoological. There are also diseases produced by minute animal organisms, such as malaria or common fever and ague. As has long been known, this disease is due to an animal parasite that infects the red blood corpuscles. It is only within recent years, however, that the entire life history of these animal parasites has been made out. As you all know, part of their life cycle is passed in a certain kind of mosquito. The disease itself has been shown to be owing to bites of these mosquitoes, and this fact pointed out the way of avoiding malaria. The ingenuous methods by means of which the propagation of mosquitoes is prevented has freed many malarious districts from pestilence. These discoveries opened the entire question of the transmission of disease by insects, and now, thanks to those brilliant observations and experiments in which some men sacrificed their lives, we know the entire life history of the microbe of yellow fever. We know it is transmitted by mosquito bites, and that disease can now be controlled. The Roman fever, once much dreaded by travelers, and the fever of the Campagna may be avoided. Thanks also to zoological studies, these diseases no longer strike in the dark. We can recognize their approach and avoid inoculation. The scourge of the sleeping sickness that attacks the people of the Congo district is due to an animal parasite. The terrible scourge of syphilis has recently been traced to a minute organism that is probably