56 HEREDITARY CHARACTERS 



destined to produce the soma or body. 1 Some of the pro- 

 geny of the one cell remain comparatively undifferentiated 

 until the time when the mature sexual elements are to be 

 produced. The progeny of the other cell become highly 

 differentiated, and form the different tissues of the body. 

 The germ-cells, from the earliest stages of development, 

 contribute nothing to the support of the body, and live 

 upon it in a parasitic manner. The soma, or body, on the 

 other hand, serves as a support and protection to the germ- 

 cells. The line of cell generations passing throughout any 

 number of generations of individuals is absolutely direct 

 as far as the germ-cells are concerned ; they, or rather their 

 contained germ-plasm, being handed on from generation 

 to generation of new individuals. The cells forming the 

 soma are, however, destined to certain death within a 

 measurable space of time. On this hypothesis Weismann 

 concluded that modifications produced by environment in 

 the body of a multicellular organism could not produce 

 any change in the germ-plasm itself, that natural selection 

 acted upon variations occurring in the germ-plasm, that all 

 new inborn characters were due to such changes, and none 

 of them to changes produced in the body by stimuli, or 

 any other external influence. 



Oscar Hertwig has put forward the theory that the germ- 

 plasm, or the power of producing germ-cells under certain 

 conditions, is retained by all the cells of the body. 2 Ap- 

 parently something of this kind must exist in the case of 

 many plants, for in them even a small portion of a leaf in 

 some cases will, when separated, grow into a whole new 

 plant and produce germ-cells. It is obvious, therefore, that 

 the cells in the portion of leaf separated from the original 

 plant must have possessed the potentiality of producing 



1 Weismann's theory does not necessarily assume that one of the two cells 

 produced from the fertilised ovum produces nothing but germ-cells, but that all 

 are produced from among the progeny of one, none from the other. 



2 Hertwig, 0., The Biological Problem of To-day, English translation, 

 Heinmann, London, 1896. 



