DEVELOPMENT OK ZOOLOGY 52? 



could not be transmitted to the next generation. Only those 

 changes which take place in the germ plasm cotild be inherited. 

 He advanced the idea of a continuous germ plasm from which 

 body after body arises in successive generations. Weismann 

 held therefore that Lamarck's supposed factors could have 

 nothing to do with evolution. Their influence stops with the 

 body of the individual. He gave more value to natural selection 

 than even Darwin. He emphasized also the union of two lines 

 of germ plasm at fertilization as the principal source of 

 variations. 



Gregor Mendel (1822-1884) through his breeding experi- 

 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. In 

 other words the offspring of parents which show bodily fluctua- 

 tions do not themselves vary about this new point as a mean. 

 DeVries believes that only "mutations" (discontinuous varia- 

 tions) arising in the germ plasm are subject to inheritance. 

 There is however probably no necessary conflict between the 

 basic 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 

 this is as potent as its discoverer imagined. For example, 

 even if natural selection does not operate to accumulate fluctua- 

 tions, it doubtless does preserve suitable mutations. 



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. 



