130 NEW YORK STATE MUSEUM 



If we try to reduce this multiplicity of factors to a few con- 

 trolling agents, we find that these are the fixation of overtaken and 

 postclimacteric types, the presence of stable physical conditions, 

 and zvithdrazval in various ways from the fields where the struggle 

 for existence is fiercest. The stable physical conditions have been 

 found by many in the open ocean, b)^ some in the deeper littoral 

 regions of the oceans, by others again in subterranean fields, by 

 some in the rivers and lakes of continental regions that remained 

 undisturbed from folding. Withdrawal from the struggle for 

 existence with other organisms has been accomplished by a variety 

 of means, as by isolation, burrowing life, small, inconspicuous size, 

 superior, often deadly, offensive and strong defensive arms, 

 through restriction to poor fare, great power of endurance, etc. 



Analysis of Biologic Factors of Persistence 



In any analysis of the biologic factors that have permitted per- 

 sistence in the tremendous stream of organisms that has evolved 

 since the Paleozoic age, we must distinguish between the two 

 entirely different groups of persistent types mentioned repeatedly, 

 (i) the postclimacteric types, and (2) the primitive central 

 vigorous stocks. 



Stable physical conditions and withdrawal from the arena of 

 the struggle for existence, as far as. possible, are the only means 

 for the salvation of the postclimacteric persistent types; the primi- 

 tive stocks, however, which persist are frequently dominant in the 

 very seats of war. While the first group are among the 

 " overtaken," and are mostly the gerontic groups which, though 

 once the most vigorous of their race, are affected with a sort of 

 congenital weakness or diminished vitality- which renders them 

 less fit for the struggle for life, the second group frequently con- 

 tains what Beecher has termed " radicles " or the stocks from which 

 the specialized groups continually branch off, while they live on 

 vigorously though primitively. We will term the first persistent 

 terminals the others persistent radicles. The first class has reached 

 fixity; the latter retained variability. 



A partial explanation of the possibility, of the existence of two 

 such radically different groups of persistent types we may find in 

 the views of Simrott.^ According to this author: " Organic evolu- 

 tion is dependent on the action of two opposing factors : that of 



. W. Simrott. The Fixation of Character in Organisms. Tlic American 

 Naturalist, 47 : 705-29. 1913. 



