ANCENLURY (OF PROGRESS LN: PALEONTOLOGY 503 
geologic periods, in so far as they are really natural divisions of 
geologic time,’ are practically contemporaneous. It is also 
believed that about three divisions in each period may be usually 
detected with much certainty throughout the world, although the 
constitution of the faunas may vary to a considerable degree. 
It becomes the task of the paleontologic geologist to inves- 
tigate the fossil faunas, the organic societies of past time. 
Through a study of the geographic distribution of species and 
genera and their relationships to preceding and succeeding 
forms, the approximate limit of the zodlogic provinces of past 
time are determined, and from these data approximate restora- 
tions of the ancient geography are made. From a study of the 
changing physical conditions and the changing faunas are deter- 
mined the migrations which we now know to have taken place 
and which Huxley suggested as a cause of sudden changes of 
fossil faunas. 
Among fossil faunas as a whole there are two conspicuous 
types, (1) the cosmopolitan faunas and (2) the provincial faunas. 
At different periods of the earth’s history these two types of 
faunas have been dominant. During Silurian time, for instance, 
there seems to have been in existence a great cosmopolitan, 
shallow-water, marine fauna, which is represented in America, in 
Europe, in Australia, and New Zealand, and probably will be 
discovered also in the other continents. The conditions for the 
development of such a fauna seem to have been the presence of 
widespread, shallow seas upon the continental platforms, epi-con- 
tinental seas, as they have recently been called.*? With broad 
seas of this sort around the borders of the continents, and 
extending into the interior by means of great tongues or lobes, 
and with conditions approximating base-levels upon the land, 
there would be a great limestone forming epoch, and with the 
probable atmospheric conditions of such an epoch3 the tempera- 
™See “The Ulterior Basis of Time Divisions and the Classification of Geological 
History,” by T. C. CHAMBERLIN, JouR. GEOL., Vol. VI, p. 449. 
2Jour. GEOL., Vol. VI, p. 602. 
3JourR. GEOL., Vol. VI, p. 609. 
