854 



THE AMERICAN NATURALIST 



[Vol. XL 



As against these purely hypothetical considerations paleontology 

 shows that during Pliocene and Pleistocene times the slow-breed- 

 ing Proboscidia evolved quite as rapidly, if not more rapidly, 

 than the rapid-breeding Rodentia. 



The Ixadaptation of Extreme Specialization 



Exfremc Specialization. — Specializations among the quadru- 

 peds take many forms: the loss of parts which under changing 

 cotiditions might be useful. In the case of the Titanotheres the 

 dmnmslKMl size of the cropping teeth, which are either degenerate 

 or wanting, is an instance. The animals while capable of browsing 

 were incapable of grazing, so far as we can infer from the general 

 presence of well adapted paired or single cropping teeth in the 

 surviving ruminants and horses. It may be said that the Titano- 

 theres had lost all cropping power through the degeneration, 

 simplification, or absence of the incisor teeth. 



Survival of the Unspecialized. — This is a general but not 

 universal principle. Cope observes: 



"Agassiz and Dana pointed out this fact in taxonomy, and I 

 expressed it as an evolutionary law under the name of the 'Doctrine 

 of the riis[)e(ialize(l.' This describes the fact that the highly 

 developed, or sj)ecialize(l types of one geologic period have not been 

 the parents of the types of succeeding periods, but that the descent 

 has been derived from the less specialized of preceding ages. No 

 better example of this law can be found than man himself, who 

 preserves in his general structure the type that was prevalent 

 during tlie Kocene j)eriod, adding thereto his superior brain- 



"The validity of tiiis law is due to the fact that the specialized 

 types of all periods have been generally incapable of adaptation 

 to the changed conditions which characterized the advent of new 

 periods. Changes of climate and food consequent on disturbances 

 of the earth's crust have rendered existence impossible to many 

 plants and imin.al>, iHid haw rendered life precarious to others. 

 Such changes have been often especially severe in their effects on 

 species of large size, which required food in large quantities. The 



