PRINCIPLES OF VERTEBRATE MORPHOLOGY 15 



character, and their origin as a sort of response to the demands of 

 certain environmental conditions. Senescent characters, although 

 sometimes apparently adaptive, are frequently valueless or distinctly 

 disadvantageous to their possessors, and have often been responsible 

 for racial extinction. The various types of adaptive complexes and 

 the several criteria of racial senescence must receive separate consid- 

 eration. 



SPECIALIZATIONS AND ADAPTATIONS 



It has been pointed out that a primitive group, after weathering a 

 radical world change and thus surviving its more highly specialized 

 relatives, tends to begin a new adaptive deployment and to occupy all 

 of the available life zones. In order to enter specialized life zones or 

 live upon very restricted types of food, adaptive specializations must 

 occur that fit various offspring of the primitive stock to occupy these 

 restricted life conditions. 



In general it may be said that the active, predaceous life is primi- 

 tive as compared with the more passive herbivorous life and that, 

 in the case of vertebrates at least, the generalized types are for the 

 most part active and predaceous, characterized by quick action, 

 keen sensitivity, sharp grasping teeth, claws as opposed to hoofs, 

 and normal balance of head, trunk, and tail. Bottom-feeding 

 types in the waters and browsing, herbivorous types on land are 

 characterized by slow action (though many herbivorous types have 

 become secondarily cursorial), cutting and grinding teeth, hoofs in- 

 stead of claws, and lack of balance in bodily proportions. 



Many of the senile races are characterized by the possession of 

 armatures of various sorts that are unquestionably of great defensive 

 value to sluggish and otherwise defenieless creatures. In spite of the 

 value of such structures to their possessors, they are rather to be 

 thought of as the products of racial aging than as responses on the 

 part of the organism to life needs. Only in a very limited sense, then, 

 can such structures be classed as adaptations, for many bony and scaly 

 excrescences of senile types have gone far beyond the bounds of use- 

 fulness and have been a mere burden and useless incumbrance to 

 their owners. 



In the body of this volume many examples of adaptive speciali- 

 zations and equally numerous illustrations of the structures resulting 

 from racial senescence will be cited and commented upon. In this 



