WBISMANNI8M AND OTHER THEORIES 403 



taken place. Hence the germ cells are really produced by the germ cells 

 of the previous generation and not by the individual's own soma (body) 

 at all: they are present from the beginning of development with the full 

 hereditary outfit, and by a few equational divisions they give rise to the 

 gametes. This is represented in Fig. 157, B. In the more usual case of 

 those animals and plants in which the germ cells appear later in the onto- 

 geny Weismann held that, although a sorting out of the units occurs in the 

 majority of the cells during ontogenesis, those meristematic cells which 

 constitute the chain connecting the fertilized egg with the germ cells 

 the germ track (Keimbahn) maintain the undiminished germ-plasm 

 (Fig. 157, C). Thus in this case as in the other there is a continuity of 

 the germ-plasm, if not a continuity of the germ cells (unless meristematic 

 cells also be regarded as germ cells). Since the germ-plasm of any 

 generation is derived directly from that of the preceding one, it is continu- 

 ous through an unlimited number of generations; and the successive somas 

 (bodies) are, so to speak, side branches given off at intervals from the main 

 stream of the germ-plasm. 



In elaborating the above views Weismann (1885, 1892) insisted 

 strongly upon the independence of the "potentially immortal" germ- 

 plasm and the transient and mortal soma 1 . He argued that since there 

 is no contribution of hereditary elements from the soma to the germ cells, 

 somatic changes being in no way impressed upon the germ cells from 

 which the next generation is to arise, there can be no inheritance of 

 acquired somatic modifications. In multicellular animals the only 

 inherited variations are those originating in the germ-plasm of the germ 

 cells or germ track as responses to internal (nutritive etc.) or external 

 environmental stimuli, or as the result of recombinations of hereditary 

 units at the time of fertilization (amphimixis). Weismann admitted 

 that the germ-plasm, though remarkably stable, might be altered directly 

 by the environment or even by modifications in the surrounding soma; 

 but he denied that in the latter case the alteration would be of such a 

 nature as would cause the reappearance of the same somatic modifica- 

 tion in the next generation. With Weismann, as with Mendel, the main 

 problem of heredity was not to discover how the characters of the organ- 

 ism get into the germ cells which it produces, but rather how the char- 

 acters of an organism are represented in the germ cell from which it is pro- 

 duced (Darbishire 1911, Chapter 12). He attempted to show how it is 

 that the stream of germ-plasm on the one hand maintains a stability suffi- 

 cient to account for the resemblance between the successive bodies spring- 

 ing from it at intervals, and on the other hand undergoes orderly changes 

 responsible for the evolutionary advance shown in a long series of 

 generations. In the words of Agar (Bower, Kerr, and Agar, 1919), 

 "According to Darwin, parents truly transmit their characteristics to 

 1 See discussion of senescence in Chapter VII. 



