406 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



color. Those of the Upper Devonian which hold marine fossils begin 

 with rocks not unlike in color to the Middle Devonian, but pass in the 

 Chautauquan (Catskill-Chemung) into dominantly olive shales. There 

 was, consequently, larger opportunity for oxidation and elimination of 

 carbon, implying a greater dominance of dryness in the climate, it was 

 a condition which the dipnoans and ganoids could partially meet, but for 

 which the selachians were inherently unfit. This difference in power to 

 utilize atmospheric oxygen, corresponding with the difference in need of 

 it in the two environments, makes the probability strong that these were 

 the internal and external factors which directed the different destinies of 

 these subclasses of fishes. 



During the Silurian the aridity was more intense than in Upper Devo- 

 nian times. The elasmobranchs then, however, were yet too small and 

 feeble to escape from the unfavorable environments of the land by taking- 

 possession of the sea. They must have suffered a severe repression, living 

 chiefly in the limited permanent areas of fresh or brackish waters, unable 

 to establish themselves in the open sea, even if some few did press away 

 living or float away dead from the margin of the land. In the land 

 waters, subjected to strong seasonal drying, a severe pressure of environ- 

 ment must have favored those fishes able to make a supplemental use of 

 air. It would seem that at this time the earliest of ganoids probably 

 arose, distinguished from their elasmobranch ancestors by the possession 

 of a primitive air-bladder. As illustrated by the evolution of many 

 groups, the development of a new order of animals apparently takes place 

 in a limited region. They must become formidable and numerous before 

 spreading widely. Thus, although the time of origin of ganoids was 

 probably as far distant as the Silurian period of aridity, their period of 

 initial expansion was deferred until the Middle Old Red Sandstone ; their 

 period of complete dominance within the land waters was deferred until 

 they were favored by the climatic changes to higher aridity, which marked 

 the closing epoch of the Devonian. 



LIVING DIPNOANS AND CROSSOPTERYGIANS 



A few lingering representatives of the dipnoans and crossoptervgians 

 still live in the fresh waters of the tropics. They throw valuable con- 

 firmatory light on the Devonian conditions of environment. These living 

 relics of the past preserve, furthermore, to modern times the destructible 

 tissues of Devonian fossils, and even, to a greater or less degree, those 

 abstract summations of life — the habits by which the Devonian fishes met 

 the conditions under which they lived. A review of these adaptations to 

 environmental relations, as seen in the living representatives, will here 

 be given. 



