Summary. 103 



Lastly, and as regards the multicellular organisms, 

 it is evident that Weismann's essay On the Significance 

 of Sexual Reproduction in the Theory of Natural 

 Selection must be cancelled. For, apart from the 

 contradictory manner in which this matter has been 

 stated (pp. 70, 93, notes), and apart also from the con- 

 sideration that other and quite as probable reasons 

 have been suggested for the origin of sexual repro- 

 duction, there is the fact that Weismann's theory is 

 no longer tenable after the above destruction of its 

 logical postulate in the absolute stability of germ- 

 plasm. For, in the absence of this postulate, there is 

 no basis for the theory that admixtures of germ- 

 plasms in sexual reproduction furnish the sole means 

 whereby heritable variations can be supplied for the 

 working of natural selection. 



J i* 



Summary. 



The theory of germ-plasm is not only a theory of 

 heredity : it is also, and more distinctively, a theory 

 of evolution. As a theory of heredity it is grounded 

 on its author's fundamental postulate— the continuity 

 of germ-plasm ; and, further, on a fact well recog- 

 nized by all other theories of heredity, which he 

 expresses by the term stability of germ-plasm. But 

 as a theory of evolution it requires two additional 

 postulates for its support — viz., that germ-plasm has 



from "the nature of the organism" no less — or even more — than from 

 "changed conditions of life." But although he appears to have enter- 

 tained the admixture of hereditary endowments in sexual unions as one 

 of the causes of variation belonging to the former category, he expressly 

 says that he did not regard it as the only, or even the main, cause. (See 

 Variation, &c, vol. i, pp. 197, 398; vol. ii, pp. 237, 252.) 



