REPORT OF THE DIRECTOR I916 131 



progressive fixation which tends universally toward greater rigidity 

 and conservatism in all characters during evolutionary advance; 

 and that of natural selection, which tends to maintain or increase 

 the variability of those characters important for survival by elimin- 

 ating individuals where such characters have become so fixed that 

 the organism fails to possess a necessary degree of adaptability." 



According to this conception it is possible that the persistent 

 types which, so far as they belong to the class of postclimacteric 

 forms or fixed terminals and are not among the dominant forms, 

 have become so fixed in all their characters as to make them per- 

 sistent, partly by the factors of progressive fixation and partly 

 by the fact that they have in various ways, as we have shown, 

 avoided the opposing factors of natural selection which otherwise 

 would have maintained their variability. Their conservation is then 

 due in part to their gerontic condition and in part to the compara- 

 tive constancy and peacefulness of their surroundings. 



In the central, frequently dominant stocks, or the persistent 

 radicles, the persistence, on the other hand, is the result of the 

 fact that through their primitive nature they are still adapted to 

 a greater variety of conditions and that while there may be and 

 probably always is considerable variation, it is around a still 

 unspecialized, primitive form and thus difficult of recognition, and 

 further that the plastic and variable characters which, according 

 to Simrott, are essentially those of size, shape, color and texture, 

 are not always quite apparent or observable to the paleontologist, 

 especially where the hard parts are not readily affected by the 

 variations in the soft parts. 



If this conception is true, only the persistent terminals will be 

 actually fixed and are true persistent types, but it would also appear 

 that the taxonomic characters, by which we recognize them as 

 persistent, are of little or no functional influence. 



Seen from another angle, that of the physiologist, the problem 

 resolves itself into one of difference of selection of adaptability. 

 Albert P. Mathews 1 writes as follows : 



In the evolution of animals two movements may be perceived : a spread- 

 ing out and a progress; a diversification and a movement forward. The 

 question which I wish to raise is whether these two movements which are at 

 right angles to each other, may not he due to the natural selection of two 

 different kinds of adaptation ; first, adaptation of form and function to dif- 

 ferent kinds of environments ; and second, the natural selection of the func- 



1 Adaptation from the Point of View of the Physiologist. American 

 Naturalist, 47: goff. 1913 



