20 VERTEBRATE ZOOLOGY 



tion. All it can do is to lose certain old characters and develop new 

 ones of a different sort. A too narrowly specialized creature is at 

 the mercy of a changing geologic age. Rather sudden climatic and 

 geographic changes have been the rule in geologic history, and when- 

 ever such changes have occurred there has been a rapid extinction 

 of the most highly specialized types, races that are no longer plastic 

 enough to adjust themselves by adaptations to the new conditions. 

 They are in a "cul-de-sac of structure," says Osborn, "from which 

 there is no possible emergence by adaptation to a different physical 

 environment or habitat. It is these two principles of too close adjust- 

 ment to a single environment and of the non-survival of characters 

 once lost by the chromatin which underlie the law that the highly 

 specialized and most perfectly adapted types become extinct, while 

 primitive, conservative, and relatively unspecialized types invari- 

 ably become the centres of new adaptive radiation." 



Racial Senescence and Degeneration 



Just as the individual grows old and suffers a retardation of all vital 

 activities, so races age and show similar evidences of lowered vitahty 

 and diminished activity. In young individuals and in young races the 

 rate of metabolism is high and the expressions of a high rate of metab- 

 olism (a more intense vitality) are seen in their comparatively active 

 life, predaceous habits, structures on the whole generalized, moderate 

 size, and lack of heavy excrescences. When the individual or the race 

 is young, the products of its metabolism are used up largely in motor 

 activities of various sorts and there is little deposition of inert mate- 

 rials such as armor, spines, heavy bones, fats, or massive flesh. 



A senescent race, on the other hand, is characterized by sluggish 

 behavior, by herbivorous habits or feeding habits involving little 

 exertion, by structures on the whole specialized or degenerate, often 

 by giant size or bulky build, and by accumulations of inert materials 

 such as armor, spines, heavy bones or flesh. These and other char- 

 acters are now very generally recognized as criteria of racial senescence. 



Structural Criteria of Racial Senescence 



Large Size. — It has been found that the highly specialized races of 

 the past have usually grown to giant size as compared with their less 

 specialized relatives. Good examples of this phenomenon are seen 

 in the great dinosaurs, and in the monster mammals of the past and of 



