REPORT OF THE DIRECTOR I916 133 



the continuous mutations of Waagen and the discontinuous muta- 

 tions or saltations of de Vries). It then appears that in these per- 

 sistent types the process of heredity has become entirely fixed and 

 rigid. While heredity is the most stable of the four processes, 

 ontogeny is the most unstable. That no changes originate from 

 this factor, requires entire stability in the habit and the use of the 



organs. 



Of the four factors, heredity, ontogeny and environment 

 evidently work toward fixity after the climacteric period, 

 since the evolutionary advance in each phylum tends to 

 result in the decrease of variability, which becomes a 

 hereditary character, and since the early ontogenetic stages repeat 

 these characters which were most conservative and firmly fixed in 

 their ancestry, this condition progresses into later and later 

 ontogenetic stages and leads thus to a loss of the potentiaUty of 

 introduction of new characters during ontogeny. The environ- 

 ment in all postclimacteric persistent types is notably stable. 



But while in persistent types heredity, environment and ontogeny 

 are rigidly stable factors — heredity especially to be considered as 

 fixed in the postclimacteric forms — selection can not be fixed but 

 is still ready to act whenever the other factors introduce slight 

 variation. It then follows that the possibility of slow development 

 continues and that all persistent types may still develop, perhaps 

 infinitely slowly. There are, then, theoretically no absolutely per- 

 sistent types, and the differences but those of rate of development. 



The difi;erence between the two groups of persistent types, the 

 relatively rigid terminals and the more variable radicles, consists 

 according to this view 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 environment, ontogeny and selection, which affect the 

 whole stock, while at the same time no changes in the germ-plasm 

 are induced through hereditary tendencies. 



General Conclusions 



The evidence here gleaned from the persistent types and equally 

 supported by both groups of persistent types, the persistent radicles 

 and persistent terminals, leads necessarily to the general conclusion 

 that there is no inherent propelling force of variation or of 

 development, and that all evolution in the last analysis is largely 



