NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 1 17 



more spiny, highly ornamented, abnormal, bizarre forms appeared at or about the 

 time when the vitality of the type was apparently declining.' 



"Furthermore, it is now commonly agreed that all groups have been most 

 plastic near their point of origin, or, in other words, that during their early history 

 all the important or major types of structure have been developed. Their subse- 

 quent history reveals the amount of minor differentiation and specialization they 

 have undergone during their period. Apparently most of the early impulses of 

 growth, whether from the environment or from vital forces, resulted in physiological 

 changes producing fundamental variations in function and structure. The later 

 influences, of environment and growth force, are expressed in peripheral differentia- 

 tion, and show that the racial or earlier characters had become fixed, and that the 

 later or specific features were the chief variables. The stimuli which, during the 

 early life history of a group, were expended in internal or physiological adjustments 

 later produced external differentiation, and in this differentiation spinosity is the 

 limit. The presence of spines, therefore, indicates the fixity of the primary physio- 

 logical characters, together with the consequent inability of the organism to change 

 due to its decrease in vitality." 



Two things are very noticeable in these quotations, which express the 

 tenor of the whole article. First, that Beecher believed that spines may be 

 useless structures, and, second, that he believed that they occur when an 

 animal has reached its maximum of vitality. I believe that the develop- 

 ment and extinction of the Dimetrodon-Wke forms is amply explained by 

 the hypotheses of Beecher and the author set forth above. 



In an unsigned review of Dr. Williston's American Permian Vertebrates, 

 and the monographs by the author on the Cotylosauria and Amphibia of the 

 Permo-Carboniferous of North America, the Geological Magazine (Nov.- 

 Dec, 1912, p. 519) says: 



"These animals {Cacops, Dimetrodon, and Edaphosaurus), and many others, 

 seem to exhibit all the features which usually occur in the last individuals of a race ; 

 they are phylogerontic, and it is improbable that they left any descendants. The 

 Texan Amphibia and Reptiles are, then, precociously specialized examples of the 

 earlier stocks which in South Africa slowly developed along many lines and gave 

 rise to the mammals among other groups. Diplocanlus may be a terminal member 

 of the line of the Microsauria, represented by Ccratcrpcton and Diccratosaurus, 

 which occur in Europe and North America." 



ORIGIN OF THE FAUNA. 

 REPTILES. 



Williston'' has recently stated his opinion that the old name Theromorpha 

 (correctly Theromora) should be reestablished to include "all the American 

 therocrotaphic forms at least," and by implication he includes the South 

 African Tkerapsida (Am. Perm. Vert., p. 71). Broom'' earlier made similar 

 suggestions in that he insists on the close genetic relationship of the African 

 Tkerapsida and the American Pelycosaiu-s, but as I tmderstand him, he does 

 not suggest their tmion in the group Theromora. 



• Williston, Science, vol. 33, p. 632; Am. Perm. Vert., p. 71; Jour. Morph., vol. 23, p. 639 et seq. 



^ Broom, Trans. Roy. Soc. S. Af., vol. I, 1910, p. 473; Bull. Amer. Mus. Nat. Hist., 1910, July, p. 197. 



