DEVELOPMENT OF ZOOLOGY 517 



ments produced facts which strongly support Weisman's idea 

 of continuity of germ plasm. 



DeVries and Johanssen, living biologists, have strongly 

 called in question Darwin's explanation by showing that the 

 small fluctuations (continuous variations) of body which Darwin 

 emphasized are not disposed to accumulate by selection. De- 

 Vries believes that only "mutations" (discontinuous variations) 

 are subject to inheritance. There is however probably no neces- 

 sary conflict between the discoveries of Darwin, Weismann, 

 Mendel, DeVries, Johanssen. When all the facts are known we 

 shall probably find that all of them have found some truth, 

 and that none of them is as potent as its discoverer imagined. 



521. Applications of Biology. From the earliest days when 

 Biology was a mere appendage to medicine and of the act of 

 living, it has been making continuous contributions to human 

 welfare. It has given us more knowledge of the health and 

 working of our own body; it has made possible the wonders of 

 surgery; it has enabled us to master many of the contagious 

 and infectious diseases of men and animals ; it has taught us how 

 to culture, breed, and select plants and animals for our uses, 

 and to prepare their products to best advantage. A very large 

 proportion of human industry depends directly upon Biology. 



Some of the steps in the progress of medicine have been 

 outlined heretofore. Many of the early biologists we have seen 

 to be physicians. 



Leeuwenhoek, one of the early users of the microscope, dis- 

 covered bacteria in 1687. Soon after this it was first suggested 

 that microscopic forms might have something to do with con- 

 tagious diseases. In 1840, Heale took the position that all such 

 diseases are caused by germs. 



Louis Pasteur (1822-1895), a Frenchman, was one of the 

 most remarkable scientists of all time. He discovered that 

 fermentation is due to microscopic organisms (1857) ; established 

 the actual connection between certain germs -and the diseases 

 produced by them; showed that germs could be made less 

 harmful by being grown in certain conditions, and that these 

 attenuated germs would produce only a mild form of the 



