392 ELEMENTS OF BIOLOGY 



Weismann. Theories of the origin of inheritable variations, 

 which both Lamarck and Darwin agreed are the basis of evolution, 

 are chiefly associated with the names of August Weismann and 

 Hugo de Vries. The theory of Weismann (1904) focuses attention 

 on the germ cells as the source of all inheritable variations. Weis- 

 mann's experimental work in which he showed that a mutilation 

 carried on through many generations did not become inheritable, 

 led him to the conclusion that the somatic cells of the body do not 

 aflfect the inheritable characters that are transmitted through the 

 germ cells. His evidence went far to refute the Lamarckian doctrine 

 of the inheritance of acquired characters and to direct attention to 

 germinal changes, occurring independent of any somatic influence, 

 as the source of variations that are inheritable and hence responsible 

 for evolutionary changes. The evidently complex nature of germ 

 plasm involved Weismann in a series of correspondingly complex 

 assumptions concerning the make-up of germinal material. This 

 aspect of his theory does not concern us here. The essential feature 

 of his doctrine is that the germ plasm forms a direct path from one 

 generation to the next, that germ cells do not arise from somatic 

 cells but only from germinal material, and that evolution is brought 

 about by germinal variations completely independent of somatic 

 modifications. 



De Vries. Hugo de Vries in 1901 published his theory of muta- 

 tions. Whereas Darwin's theory holds that small variations are the 

 basis of evolution, De Vries by experimental work demonstrated 

 that wide differences between parent and offspring sometimes occur. 

 These mutations, as they are now called, are more frequent than is 

 apparent to the casual observer. Records show that a considerable 

 number have occurred among domesticated animals, for example, 

 hornlessness in cattle and the peculiar conformation of Merino 

 sheep. In laboratory animals mutations have occurred with consid- 

 erable frequency. In Drosophila, a small fruit fly much used in 

 experimental genetics, several hundred different mutants have ap- 



