Weismannism up to date (1893). 121 



main facts of heredity, and allied phenomena, admit 

 of being explained if once the postulate be accepted. 

 If this were urged, however, I should have two remarks 

 to offer. The first is that Weismann, in constructing 

 his ideal mechanism, has gone very much further in 

 the way of elaboration than can possibly be required 

 for this purpose. So much further, indeed, that his 

 purpose has evidently been the constructing of his 

 ideal mechanism, as I have just said, for its own sake, 

 and not for the sake of substantiating its basal pro- 

 position by showing how well the latter can be made 

 to work in explaining the phenomena of heredity, &c. 

 Moreover — and this is my second remark — however 

 well the basal proposition may be made to work in 

 this respect, we must not be deceived into supposing 

 that such a fact is equivalent to a substantiation of 

 the proposition. This proposition — the continuity of 

 germ-plasm — is the inverse of that which constitutes 

 the basis of the theory of pangenesis. For while the 

 latter assumes that in the last resort it is always 

 somatic tissues which produce the substance of 

 heredity, the former simply inverts the terms of this 

 assumption, and holds that it is always the substance 

 of heredity which produces the somatic tissues. Now, 

 in all cases where one theory consists in thus simply 

 inverting the terms of another, it will be found that 

 the facts which they both- seek to explain lend 

 themselves equally to explanation by either, up to 

 some certain and usually distant point, where a crucial 

 test becomes possible. Take, as an example, the 

 geocentric and heliocentric theories of the solar 

 system. Here the question was whether the earth 

 moved round the sun, or vice versa ; and so many of 



