Appendix I. 181 



is that his a priori argument from " inconceivability " 

 cuts both ways, and that it makes at least as much against 

 germ-plasm as it does against gemmules. Therefore, 

 having now considered what Weismann has said against 

 the conceivability of gemmules on grounds of general 

 reasoning, I shall proceed to show that quite as much — 

 or even more — may be said in the way of a tu quoque. 

 In other words, we have now finished with the second of 

 the three propositions w^hich we are examining (see p. 71), 

 and proceed to our consideration of the third. 



First of all, I do not see any greater difficulty in 

 supposing that the " carriers of heredity " proceed centri- 

 petally from somatic-cells to germ-cells, than in supposing that 

 they proceed centrifu gaily from the germ-cells to the somatic- 

 cells which they are engaged in constructing. Nor do I 

 see any more difficult} in imagining these "carriers of 

 heredity" to be capable of constructing a new organism 

 if they have first proceeded centripetally, and are thus 

 severally representative of all parts of the parent organism 

 after its construction has been completed, than I do if they 

 have proceeded centrifug.illy, and are thus similarly repre- 

 sentative of all parts of that organism before its construction 

 has been commenced^-. 



1 Weismann speaks disparagingly of Darwin's theory as a ''theory 

 of preformation" (p. 316). "We must assume," he adds by way of 

 explanation, "that each single part of the body at each developmental 

 stage is, from the first, represented in the germ-cell as distinct particles 

 of matter, which will reproduce each part of the body at its appropriate 

 stage as their turn for development arrives." But must we not likewise 

 "assume" exactly the same thing in the case of Weismann's own 

 theory? To me, at any rate, it appears that the description is quite as 

 appropriate to germ-plasm as it is to gemmules. Nor can I see any 

 distinction, even where he seeks to draw it more expressly, as for 

 instance — "Every detail in the whole organism must be represented in 

 the germ-plasm by its own special and peculiar arrangement of the 

 groups of molecules. . . . not indeed as the preformed germs of structure (the 

 gemmules of pangenesis), but as variations in its molecular constitution." 

 [Essays, p. 194.] Again, on page 325 he gives a foot-note explaining 



