354 THE POPULAR SCIENCE MONTHLY 



fossil forms to living forms and he has succeeded in tracing the remote 

 ancestry of a number of living races of Tna.Trmrm.1s. 



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 or- 

 ganization 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 intellec- 

 tual 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. Eemote 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 establish- 

 ing 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 cor- 

 puscles. 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 Eoman 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 inocu- 

 lation. 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 



