PRINCIPLES OF VERTEBRATE MORPHOLOGY 21 



the present, such as the proboscidians and the whales. In these forms 

 growth has run riot, probably because of the lack of growth-inhibiting 

 factors. 



Spinescence. — In both vertebrates and invertebrates the develop- 

 ment of spines, horns and other chitinous or bony excrescences are 

 generally believed to be evidences of racial old age. As the general 

 metaboUsm slows down there appears to be a tendency for hard or 

 dead substances to be deposited in regions of lowest metabolic rate, 

 just as debris collects in an eddy. Good examples of this phenomenon 

 are found in Stegosaurus, in Edaphosaunis, in the homed toads, in 

 many teleost fishes, in porcupines, and in the deer family. In the last- 

 named group the extinct Irish elk stands out as a classic example of 

 a type that went a step too far in the elaboration of excrescences, and 

 became extinct. 



Degeneracy. — One of the systems most commonly found in a de- 

 generating condition is the dentition. Many of the most highly 

 speciaUzed groups have undergone a partial or total loss of teeth. 

 This is true of the edentates (archaic types of placental mamals), of 

 the turtles (among the oldest and most specialized among reptiUan 

 orders), of the sturgeon (a degenerate end product of a long line of 

 chondrostean fishes), and of modern birds (possibly the most highly 

 speciaUzed vertebrates). Loss of teeth is equally common among the 

 extinct groups of the past, such as the beaked dinosaurs, and the 

 great sauropods. Degeneration of the tail is almost as common 

 among highly specialized and senescent races as is the loss of teeth. 

 Examples of taillessness are too mmierous to list. Loss of limbs is 

 also very common in senescent types. 



The Eel-like Form. — Lull also recognizes as a criterion of racial 

 old age great bodily elongation, usually accompanied by partial or 

 total loss of limbs. Gregory lists forty-four distinct types of eel-like 

 creatures among the vertebrates, including: — three groups of cyclo- 

 stomes, one of sharks, a lung fish, three teleostome groups, three 

 amphibian groups, five reptilian groups, and one group of mammals. 

 In all cases the criteria are about the same, though the degree of elon- 

 gation and reduction of limbs varies greatly. 



Elaborate Coloration. — Most primitive or generalized groups of 

 animals have dull colors and indistinct patterns; but one of the most 

 striking features of highly specialized climax groups of vertebrates, 

 such as the teleost fishes and the birds, is their remarkably, high color- 



