Statement of Weismanris System (1886). 17 



We are now in a position fully to understand Pro- 

 fessor Weismann's theory of heredity in all its bearings. 

 Briefly stated, it is as follows. The whole organiza- 

 tion of any multicellular organism is composed 

 of two entirely different kinds of cells — namely, the 

 germ-cells, or those which have to do with repro- 

 duction, and the somatic-cells, or those which go to 

 constitute all the other parts of the organism. Now, 

 the somatic-cells, in their aggregations as tissues and 

 organs, may be modified in numberless ways by the 

 direct action of the environment, as well as by special 

 habits formed during the individual lifetime of the 

 organism. But although the modifications thus in- 

 duced may be, and generally are, adaptive — such as 

 the increased muscularity caused by the use of muscles, 

 " practice making perfect " where neural adjustments 

 are concerned, and so on, — in no case can these so- 

 called acquired, or " somatogenetic," characters exer- 

 cise any influence upon the germ-cells, such that 

 they should reappear in the next generation as con- 

 genital, or " blastogenetic," characters. For, according 

 to the theory, the germ-cells as to their germinal 

 contents differ in kind from the somatic-cells, and 

 have no other connexion or dependence upon them 

 than that of deriving from them their food and 

 lodging. So much for the somatic-cells. 



Turning now to the germ-cells, these are the re- 

 ceptacles of what Weismann calls the germ-plasm ; 

 and this it is that he supposes to differ in kind 

 from all the other constituent elements of the 

 organism. For the germ-plasm he believes to have 

 had its origin in the unicellular organisms, and to 

 have been handed down from them in one continuous 



c 



