272 THE AMERICAN NATURALIST [Vol. LVI 



persistent terminal types, on the other hand, through the 

 gerontic rigidity of the remaining stock of genes. The 

 principal causes of the persistence of terminal forms 

 would then be the failure of production of new genes 

 arising from the cytoplasm, through external influences, 

 and the senescent rigidity of the remaining genes. 



The persistent radicles, on the other hand, correspond 

 to the extreme development of what Salfeld terms " Kon- 

 servativreihen. " There are series in which the salto- 

 mutations appear in very long intervals, while the nu- 

 merous side-branches (which furnish the index-fossils) 

 develop by rapid salto-mutations. These persistent radi- 

 cles are therefore able to undergo new periods of explosive 

 and climacteric development (" Virenz-perioden " of 

 Wedekiiid) and are thus still less absolutely persistent 

 than the persistent terminals. In these conservative 

 series, according to Salfeld, flucto-mutation is so prevalent 

 that sharply defined " species," or better mutants, can 

 not be separated, as notably in the phyla of Phylloceras 

 and Lytoceras which range, qualitatively unchanged in 

 their characters, through Jurassic and Cretaceous time. 

 They thus represent true persistent radicles. This fact, 

 combined with the observation of the vitality, relative 

 primitive simplicity and adaptation to a variety of condi- 

 tions of persistent radicles, pointed out by the writer in 

 his former paper, suggests that the complex of genes is 

 able to remain relatively undisturbed through external 

 influences (only flucto-mutations appearing) in one part 

 of these groups which persist as radicles, while those parts 

 which become changed through the addition of genes by 

 way of the cytoplasm turn into the side-branches by salto- 

 mutation. 



