EVOLUTION AS A NATURAL PROCESS 145 



next and later generations, it will be evident, in the 

 first place, that the germ plasm of their nuclei is the 

 only essential substance that connects parent and off- 

 spring. This stream of germ plasm passes on in direct 

 continuity through successive generations — from egg 

 to the complete adult, including its own germ-cells, 

 through these to the next adult, with its germ-cells, 

 and so on and on as long as the species exists. It does 

 not flow circuitously from egg to adult and then to new 

 germ-cells, but it is direct and continuous, and appar- 

 ently it cannot pick up any of the body-changes of an 

 acquired nature. Now we see why individual accjuisi- 

 tions are not transmitted. The hereditary stream of 

 germ plasm is already constituted before an animal 

 uses its parts in adult life ; we cannot see how altera- 

 tions in the structure of mature body parts through use 

 and adjustment to the environment can be introduced 

 into it to become new qualities of the species. 



It must be clear, I am sure, that this theory supple- 

 ments natural selection, for it describes the physical 

 basis of inheritance, it demonstrates the efficiency of 

 congenital or germ-plasmal factors of variation in con- 

 trast with the Lamarckian factors, and finally in the 

 way that in the view of Weismann it accounts for tlie 

 origin of variations as the result of the commingling 

 of two differing parental streams of germ plasm. 



At first, for many reasons, Weismann's theories did 

 not meet with general acceptance, but during recent 

 years there has been a marked return to many of his 

 positions, mainly as the result of further cytological 

 discoveries, and of the formulation of Mendel's Law and 

 of De Vries's mutation theory. The first-named law 



