NORTH AMERICA AND THEIR VERTEBRATE FAUNA. Ill 



to any particular form. The lack of a complete, or even fairly complete, 

 armor indicates that while the predatory and harmless animals were becom- 

 ing well adjusted to each other in the relation of eater and eaten, the condi- 

 tion had not yet developed to the stage it reached among the Devonian fishes, 

 the Mesozoic Dinosaurs, or the later Tertiary mammals. 



I have not considered in this discussion that the spines of Dimetrodon or 

 Edaphosaurus were protective in character. That they may have had some 

 such function in their incipiency, and may have developed from originally 

 useful structures, is possible, but all the attempts, notably those of Jaekel 

 and Abel, to demonstrate such a protective function seem to me to be based 

 on wrong assumptions. Such spinous or tuberculated conditions of the skull 

 as are shown in Zatrachys and Chilonyx may be regarded in the light of protec- 

 tive defenses, but the forms are too little known to warrant any extended 

 speculation. They are discussed below among the over-specialized forms. 



OVER-SPECIALIZED FORMS. 



One feature of the Permo-Carboniferous vertebrate fauna which has, 

 from its striking character, attracted much attention is the presence of 

 several forms which have developed, to a remarkable degree, spines, horns, 

 excrescences, etc. The number and importance of these forms have been 

 exaggerated, because of their peculiarity, but a careful survey of the fauna, 

 especially when an attempt at restoration is made, reduces this feature to a 

 position of much smaller importance than was originally supposed. The vari- 

 ous structures are, however, of sufficient importance to warrant a discussion 

 of their possible origin and effect. 



Dimetrodon, Clepsydrops, Edaphosaurus, Platyhistrix, Diplocaulus, and to 

 a less degree Zatrachys are the most striking forms. The great horns on the 

 posterior portion of the skull of Diplocaulus and the spinous excrescences on 

 the skull of Zatrachys are different in form but similar in origin and effect to 

 the elongation of the spines in the other mentioned genera. All are instances 

 of the development beyond the point of usefulness of a structure which may 

 (must?) have been originally of some value to the animal. 



The conceivable causes for the development of such extravagant and 

 apparently useless structures are: 



(i) The overcrowding of a limited area, resulting in a severe struggle for ex- 

 istence. 



(2) A wealth of possible environments, leading to the practical isolation of 



forms that would otherwise be eliminated. 



(3) Eminently favorable conditions permitting the growth of structures far 



beyond an originally useful condition. 



(4) The disturbance of a phylum, due to approaching extinction. 



(i) The first suggestion seems to the author extremely unlikely to have 

 been the cause of the structures here considered. The struggle for existence 

 has many times resulted in the development of very peculiar adaptations. 



