THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 25 



The stages of development of the various phyla represented frequently 

 reveal differences in the time of geological horizons, otherwise indistinguish- 

 able. In the Red Beds (Permo-Carboniferous) of Texas and Oklahoma, 

 a group of vertebrates occurs in a series of deposits indistinguishable from 

 a series of beds in north central New Mexico carrying very similar verte- 

 brates, but the difference in the age of the beds is revealed by the stage of 

 evolution of the two genetically closely related groups. 



(e) Peculiarities of the Fauna. 



The peculiarities of a fauna, either aquatic or terrestrial, are revealed 

 in the structure of the individual; such peculiarities are generally in close 

 response to the conditions of life. It would require a long treatise to discuss 

 the subject with any approach to adequacy, but on the solution of a paleo- 

 geographic problem where so much must be determined by indirect evidence 

 it is necessary to exert the keenest observation to detect every suggestion. 

 All the influence of the organic and inorganic world is reflected in the 

 armor, mimetic adaptations, weapons, feeding adaptations, modes of pro- 

 gression, etc. Here the works of Abel, Walther (parts i and ii), Lull, and 

 Grabau (chap. 28) already cited are most useful. 



It is not alone in response to the environment that peculiarities appear. 

 As Beecher has shown, the approaching end of the life of a group is heralded 

 by changes of a marked character in the constituent individuals, as the 

 assumption of spines, excrescences, etc., and though he drew his illustration 

 largely from invertebrates, the same thing can be shown for vertebrates.^ 

 Provincial or cosmopolitan character in a fauna is revealed by its constituent 

 members. Peculiarities of development shown in minor unique variations, 

 strongly accentuated peculiarities of structure, or even a dominant character 

 or direction of variation in the whole fauna, as thinness or thickness of shell, 

 pauperization, etc., all point to a provincial character. The discovery of 

 such characters would at once direct attention to the examination of all the 

 other observable facts concerning the unit to test the suggestion of local, 

 isolated deposits, as in a region of gulfs or bays, lagoons, and small inland 

 seas, or, for terrestrial forms, isolated peaks, valleys, patches of woodlands, 

 oases in a desert, etc.^ 



A greater community of structure in the fauna with less evidence of 

 peculiarities not (immediately) explainable by use suggests wide areas of 

 similar conditions where the friction of readily commingled forms from far- 

 separated areas tends to maintain the mean of life. Such are the marine 



1 Beecher, C. E., Origin and Significance of Spines: A Study in Evolution. Amer. Jour. 



Sci., vol. VI, 4th series, 1898. 

 Case, E. C, The Permo-Carboniferous Red Beds of North America and Their Vertebrate 



fauna. Carnegie Inst. Wash. Pub. No. 207, p. iii, 1915. 

 ^ The isolation of the group may be accomplished by more obscure factors than the purely 



geographical, as temperature, pressure, strength of waves, etc. 



