846 



THE AMERICAN NATURALIST [Vol. XL 



(1) Since the publication of The Origin of Species naturalists 

 have disagreed as to one of Darwin's main propositions, namely, 

 that the struggle for existence is so intense that variations adaptive 

 or inadaptive, no matter how slight, will tend respectively toward 

 survival or elimination. This raises the question of the modes 

 of evolution, of character or organ building, in mammals, which is 

 treated elsewhere. 



(2) Whatever may be true as to the above feature of the 

 selection theory, there is a general consensus of opinion that ani- 

 mals which 'present the highest adaptive combination of favorable 

 characters, of fully formed organs, and the highest adaptability 

 or capacity of favorable change of habit or structure, will tend to 



(3) Similarly there is a consensus, from certain repeatedly 

 observed facts in paleontology that in varietal, specific, generic,, 

 family selection, not only adaptive or inadaptive combinations 

 of characters but also single fidly formed organs, such as the brain, 

 the limbs, and the teeth, have in course of time been the causes 

 of selection or extinction, partly in connection with changes of 

 environment, partly because inherently adaptive or inadaptive. 



(4) Thus we make the generalization that in certain cases 

 extreme bulk, extreme specialization, the development of certain 

 dominant characters, have led to extinction; that large-brained 

 have replaced small-brained types; that certain types of teeth 

 or certain types of limb and foot structure have simultaneously 

 over large parts of the world been found wanting and thus proved 

 fatal to their possessors. 



These are the general lines of thought which have been followed 

 by many authors since Darwin first directed our attention to this 

 subject. It is necessary, however, to look into these causes some- 

 what more critically since many of them have been assumed without 



Inadaptive Foot and Molar-Tooth Structure — Waldemar Kow- 

 alevsky, the Russian paleontologist, was one of the pioneers in 

 this line of reasoning. He observed in his great monograph ^ 



■"Monograph der Gattimg Anthracotherium Cuv. und Versuch einer 

 nat.jrlichen Classification der fossilen Hufthiere." Palceontogrnphica, n. s., 

 vol. 2, 3. (XXII.) 



