258 THE AMERICAN NATURALIST [Vol. LVI 



(families and orders). In the latter case superior sets 

 of offensive arms and defensive armors, early developed, 

 appear to have helped to give stability to some, as in the 

 scorpions (pincers and poison glands), limnlids (leatherj^ 

 armor combined with burrowing habit and enormous 

 broods). Some, as the turtles, have successfully special- 

 ized for protection. 



In trying to reduce the multiplicity of factors to a few 

 controlling agents, it was found " that these are the fixa- 

 tion of the ' over-taken ' and post-climacteric types, the 

 presence of stable physical conditions, and withdrawal in 

 various ways from the fields where the struggle for ex- 

 istence is fiercest. The stable physical conditions have 

 been found by many in the open ocean, by 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 by fold- 

 ing. Withdrawal from the struggle for existence with 

 other organisms has been accomplished by a variety of 

 means, as by isolation, burrowing life, small, inconspicu- 

 ous size, superior, often deadly, offensive and strong 

 defensive arms, through restriction to poor fare, great 

 power of endurance, etc." 



In an analysis of the biologic factors that have per- 

 mitted persistence, two entirely different groups of per- 

 sistent types must be distinguished: (1) The post-cli- 

 macteric types; (2) the primitive central stocks. The 

 former rely on stable physical conditions and withdrawal 

 from the arena of the struggle for existence, as far as 

 possible; while the latter are frequently dominant in the 

 very seats of war. We have termed the first persistent 

 terminals, the others persistent radicles. 



The persistent terminals were considered to have be- 

 come so fixed in all their characters as to make them 

 persistent partly by the factors of progressive fixation 

 and partly by the fact that they have in various ways 

 avoided the opposing factor of natural selection; their 

 conservation thus being in fact due in part to their ge- 



