644] ARRESTED EVOLUTION 



259 



rontic condition and in part to the peacef nines s of their 

 surroundings. 



The persistent radicles, on the other hand, were thought 

 to owe their persistence to the fact that through their 

 primitive nature they are still adapted to a greater vari- 

 ety of conditions and that while there may be consider- 

 able variation, it is around a still unspecialized, primi- 

 tive form and thus difficult of recognition. 



Or, expressing the same ditference in terms of the 

 four processes of heredity, ontogeny, environment and 

 selection, around which, according to shorn, the life and 

 evolution of organisms continuously center, we found that 

 " the difference between the two groups of persistent 

 types, the relatively rigid terminals and the more vari- 

 able radicles, consists in the fact that in the former all 

 factors have become fixed and unresponsive to stimuli, 

 only the selection still slowly acting, while the latter are 

 so well adapted to a variety of conditions that no changes 

 readily originate through any of the processes of envi- 

 ronment, ontogeny and selection, which affect the whole 

 stock, while at the same time no changes in the germ, 

 plasm are induced through hereditary tendencies." 



The following notes are written with the intention 

 partly to add certain new factors that appear to con- 

 tribute to the persistence of forms, and that had not been 

 taken into account in the first essay; and partly to enter 

 deeper into the analysis of the ultimate causes of per- 

 sistence made possible through more recent investiga- 

 tions into the nature of phylogenesis. 



1. Additional Factors of Persistence 

 The new factors here mentioned have all to do with 

 the methods of reproduction whose influence had not 

 been recognized, in the first paper, in the percentage 

 table of persistent genera. 



(a) Reproduction hij Simple Division.— In the Proto- 

 zoa reproduction takes place by division without any 



