Weismannism tip to date (1893). 153 



may continually be blended in a fresh manner, but it is incap- 

 able of giving rise to new variations, even though it often 

 appears to do so. . . . The anise of hereditary variation must lie 

 deeper than this. It must be due to the direct effects of external 

 influences on the biophores a?id determinants^. 



These quotations are enough to show that Weis- 

 mann has now abandoned his original theory of 

 congenital variations being exclusively due to amphi- 

 mixis, and adopts in its stead the precisely opposite 

 view — viz., that the origin of all such variations must 

 be ascribed to the direct influence of causes acting on 

 germ-plasm from without. Up to the present year 

 the very essence of the whole Weismannian theory 

 of evolution has been that, owing to the stability 

 of germ-plasm since the first origin of sexual pro- 

 pagation, " the origin of hereditary individual varia- 

 tions cannot indeed be found in the higher organisms, 

 the Metazoa and Metaphyta ; but is to be sought 

 for in the lowest — the unicellular organisms," because 

 " the formation of new species, which among the 

 lower Protozoa could be achieved without amphigony, 

 could only be attained by means of this process in 

 the Metazoa and Metaphyta. It was only in this 

 way that hereditary individual differences could arise 

 and persist-." 



But about the beginning of the present year we 

 have this fundamental doctrine directly contradicted 

 in such words as : — 



The origin of a variation is equally independent of selection 

 and amphimixis, and is due to the constant occurrence of slight 

 inequalities of nutrition in the germ-plasm 3 . 



1 The Germ-plasm, pp. 414-415. Italics Weism arm's. 

 2 Essays, vol. i. p. 284. :i The Ger?n-plasm, p. 431. 



