502 
GEOLOGY: J. BARRELL 
Devonian, and they did not dominate the sea in such exclusive fashion 
as they dominated the continental deposits of the Lower Devonian 
until the opening of the Mississippian (Lower Carboniferous) period. 
In contrast also to the abundant ganoid fauna in the continental waters, 
only a single species of ganoid is found in the marine Ulsterian (Middle 
Devonian) rocks of the interior of North America, and they have but 
scanty representation in the seas until the Mesozoic era. This record 
appears to show conclusively that the center of piscine evolution was 
within the land waters, and that the ganoids represented a closer ad- 
justment to that environment than did the sharks. 
This brings us to the second problem, the relation of environment to 
the origin of the amphibians. The nature of the sediments shows that 
through Silurian and Devonian times the climates, although subject to 
variation, tended to be warm and semi-arid, that is, marked by a pro- 
nounced alternation of wet and dry seasons. In the middle Silurian in 
fact a high degree of aridity existed in certain regions. The initial 
development of ganoids, distinguished in their organization from sharks 
by their capacitv to supplement the use of water in respiration by the 
direct use of air in an air bladder, probably goes back to this Silurian 
epoch of aridity; since in the Middle Old Red Sandstone stage they are 
already well differentiated and expand in numerous species through the 
fresh waters. We must look to the previous period therefore for their 
origin. During the Upper Devonian the climate became more markedly 
semi-arid than in the Lower Devonian. Then the traces of sharks dis- 
appear completely from the fresh waters, but dipnoans and ganoids 
continue to exist. This migration of the sharks in habitat is logically 
to be correlated with the fact that elasmobranchs have no air bladder, 
not even in rudimentary or vestigial form, whereas the other two orders 
are so provided. It is notable that the few living species of dipnoans, 
or lung fishes, are restricted at the present time to tropical regions 
whose flowing waters are restricted to a wet season, and show such 
marked adaptations for surviving through seasons of drought that in 
those environments they possess a permanent advantage over other 
fishes. 
The fossils of fishes in the Old Red Sandstone are apt to be crowde d 
into occasional layers and are associated with bitumen, indicating in 
those layers an unusual amount of organic matter buried without being 
exposed to atmospheric oxygen. Judged by the environment, they ap- 
pear to have died in shoals owing to crowding into the foul waters of 
pools shrinking in the dry seasons. The seasons of drought formed the 
