Iiislle: 
BOURNAL OF GEOLOGY 
FULV-AGUGOST, 10909 
tH hAUNAL -REEATIONS- OF THE EARLY 
VERTEBRATES 
S. W. WILLISTON 
The University of Chicago 
VIII 
The environmental conditions affecting the evolution of the early 
air-breathing vertebrates offer at the present time many peculiarly 
difficult problems, problems which must depend in large measure 
upon the geologist for solution. They are very different from those 
which confront the student of the neozoic vertebrates, since we have 
better data for comparisons and conclusions in the living faunas as 
well as in our existing climatic and geographic conditions. And 
especially are the problems more involved and complicated when we 
attempt to deal with the marine or aquatic air-breathers of those 
early times. Here we can practically predicate little as to the con- 
ditions of the oceans and climates in which they lived. But these 
early vertebrates do offer, it seems to me, much that is suggestive 
regarding the migrations and evolution of faunas, involving theories 
as to paleogeographic conditions and changes, and, within certain 
limits, the climatic conditions which surrounded and controlled the 
migrations. And it is of this phase of the subject that I would choose 
to speak now. 
As has been said, the evidence offered by the vertebrates, when 
available, is often, if not usually, more decisive than that of any 
other class of organisms in the determination of the relationships 
and correlations of faunas. A single species of the higher vertebrates 
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