66 THE EVOLUTIONARY THEORY 



which he assailed the doctrine of transmission of 

 acquired characters, and gave carefully developed 

 reasons for a new theory, that heredity depends upon 

 the transmission of germ-plasm, originally derived 

 from the remotest ancestry, and unaffected by the char- 

 acters acquired by organisms during their individual 

 lives. Organisms are made up of minute units called 

 cells, and are developed from transmitted germ-cells 

 by means of assimilation of food, subdivision and 

 differentiation in function of the multiplying cells and 

 organs which they combine to produce. Professor 

 Weismann maintained that the original germ-plasm 

 develops into two kinds of cells — the somatic cells 

 and the germ-cells — and these are mutually isolated. 

 The somatic cells build up the individual organism, 

 and are subject to modification by the influence of 

 environment and by use and disuse of organs. But 

 as these cells are not transmitted to offspring, the 

 characters which are thus acquired cannot be trans- 

 mitted or inherited. The germ-cells alone constitute 

 the material source of new organisms, and they are 

 unaffected, because of their isolation, by the influences 

 which modify the somatic cells and the individual 

 organism during its lifetime. The arguments by 

 which this theory was supported are too complicated 

 to present adequately in these lectures,^ but their 



repeated in his Evolution Theory, 2 vols., translated by J. A. Thom- 

 son, 1904, Lees, xxiii-xxv. Cf. R. H. Lock, Recent Progress, pp. 

 59-72; Baldwin, Die. of Philos., s. v. "Acquired Characters"; H. 

 Calderwood, Evolution, pp. 40-42. 

 1 Cf., however, pp. 204-207, below. 



