PARTICULATE THEORY OP HEREDITY 235 



sense meant the sorting out of the wholes of ancestral 

 germ-plasms with which he peopled the chromosomes.^ 



The danger of any appeal to a theory of representative 

 particles obviously lies in the ease with which by its means 

 any phenomenon might be accounted for, if the theorizer 

 is allowed to endow the particles with any and all the 

 attributes that he wishes to use in his explanation. It 

 was because Bonnet, Spencer, and Weismann assigned 

 arbitrarily attributes to the ultimate particles of living 

 matter, that these views appear to-day highly speculative. 

 The different kind of evidence to which the modern theory 

 of the gene appeals is what I wish to emphasize here. 



The Evidence for the Gene 



The evidence that Mendelian inheritance rests on the 

 distribution of separate elements has already been given. 

 The numerical results obtained in the second generation 

 from any Mendelian cross involving a pair of contrasted 

 characters, find their explanation on the assumption that 

 the two original germ-plasms (or some element in them) 

 sej)arate cleanly in the germ-cells of the J^i hybrid. Tested 

 by back-crossing the assumption is verified. Recombining 

 the Pi, F^, F2 individuals in all possible ways also gives 

 results consistent with the very simple assumption that 

 whatever it is that causes one race to produce one charac- 

 ter, and another race another character, the two separate 

 in the hybrid in such a way that equal numbers of germ- 

 cells of each kind are produced. Up to this point the 

 results do not tell us whether the two germ-plasms separ- 

 ate as wholes — one from the other — or whether only some 

 part or parts behave in this way. But when two or more 



^ The nominal adoption (1904) toward the end of his career of heredi- 

 tary units in the Mendelian sense did not go deep. Weismann still adhered 

 to his view of dissociation of the ids as their most characteristic feature — 

 the only one in fact for which they were originally invented. The evidence 

 on which Mendelian units rest has nothing whatever to do with this 

 cardinal doctrine of Weismann's teaching. 



