HEREDITY AND EVOLUTION 97 



characters. The danger here, in respect of method, 

 is that we are hypothetically inferring unknown 

 organic procedure, so as to meet the demands of 

 external observation. 



Weismann enters into the hidden mechanism and 

 its movements, with the view of deciphering the 

 structural details, and the phases of activity within 

 the germ-cells, which may carry an explanation of 

 development both before and after birth. This leads 

 to the heart of the question, allowing for Darwin s 

 observations being held in reserve for a time. Weis 

 mann starts from the generally accepted positions in 

 the cell theory, here to be carefully kept in view, 

 that all life begins from germ-plasm, and assumes its 

 primary individual form in a germ-cell ; that for all 

 life, germ-cells have a common structure, so that it 

 is impossible to distinguish species by reference to the 

 characteristics of these cells ; and yet, that they have, 

 in their distinctive constitution, the germinal provision 

 for all that belongs to their species. As to the origin 

 of these germ-cells, the ordinary hypothesis assumes 

 that the organism produces germ-cells afresh, again 

 and again, and that it produces them entirely from 

 its own substance. 1 Weismann denies this. Con 

 centrating on the severance of these germ-cells from 

 the other cells of the body, and upon the distinctive- 

 ness of their function as reproductive cells, he adopts 

 the hypothesis of the continuity of the germ-plasm 

 as the causal energy giving rise to individual life in 

 successive generations. Jager, in discussing Pan- 

 genesis, had spoken of the reservation of germi 

 nal protoplasm. The alternatives apparently open, 



1 Essays upon Heredity, vol. i. p. 174. 

 G 



