270 THE AMERICAN NATURALIST [Vol. LVI 



involved in their production. Many of these were found 

 to be connected with the environment, others, acting 

 through variability, or its lack, with selection, and still 

 others with the processes of heredity and ontogeny. 

 While of these four fundamental processes of evolution, 

 viz., heredity, ontogeny, environment and selection, that 

 of selection may account for the cases of persistence 

 wliere variability has been reduced to a minimum, possi- 

 bly by the lower modes of propagation mentioned above, 

 and that of environment accounts for persistence in those 

 cases where the environment has become so stable as to 

 lack the actual stimulus for further development, it is 

 obvious that still more important factors are involved 

 in heredity and ontogeny that make for persistence in 

 organisms, especially as it is seen in the post-climacteric 

 forms, or persistent terminals. Both the conservative 

 process of heredity and the much less rigid one of on- 

 togeny appear to become more or less tixed and inacces- 

 sible to changes in persistent types. 



None of these four processes gives any clue to the 

 actual mechanics of the factors that induce persistence. 

 In tr\dng to trace the latter to its ultimate causes, it be- 

 comes, therefore, necessary to go beyond these processes, 

 and to appeal to the important conclusions that have 

 been obtained by modern experimentation and observa- 

 tion regarding the methods of inheritance and production 

 of new characters by means of the genes or character- 

 determiners of the heredity-chromatin. 



Among these conclusions especially suggestive in regard 

 to our problem, are the views advanced by Diirken and 

 Salfeld.^ These authors have, one through an analysis 

 of all recent zoological experiments on evolutionary prob- 

 lems, the other through a corresponding analysis of the 

 evolution among the fossil ammonites, arrived at the view 

 that variability or the appearance of new characters, and 

 of new combinations of characters is produced in differ- 



6 Diirken, B., and Salfekl, H., "Die Phylogenese. Fragestellungen zu 



