468 HISTORICAL GEOLOGY 



that at certain times evolution proceeded at an unusually rapid rate. 

 The difference in the faunas of succeeding stages is due to several 

 causes. At the beginning of the period there were a number of em- 

 bayments so isolated that the evolution of the faunas of each pro- 

 ceeded independently, until each possessed many characteristic and 

 peculiar species. As the seas spread over the land later in the period 

 these embayments were, one after another, joined together, and as 

 quickly as a waterway opened species from each embayment spread 

 to the others, and a struggle for existence resulted which produced 

 rapid and marked changes in the life, exterminating many species. 

 The conflict thus brought about also caused the rapid rise of new 

 forms not found in any of the original faunas. 



The changes in the physical conditions were another cause of rapid 

 evolution. As a result of the extension of the epicontinental seas, 

 new food was doubtless introduced and currents were developed 

 which may have brought about changes in temperature. 



It should not be forgotten in this connection, however, that the 

 differences in the faunas of beds of nearly the same age may be due 

 entirely to the fact that one bed was deposited, for example, in shallow 

 water and consequently had a shallow water fauna, and another in 

 deep water and had a deep water fauna. The life of two such beds 

 may, consequently, differ more widely than those of very different 

 ages which were deposited under similar conditions. 



Climate and Duration. — Little more can be said of the climate of 

 the Devonian than of the Cambrian and Ordovician, and the evidence, 

 as in the latter, points to a uniformly warm climate over the entire 

 world. In certain places deserts existed as now, while in others ex- 

 tensive swamps were present. 



The period was probably little more than half as long as the Ordo- 

 vician. 



REFERENCES FOR THE DEVONIAN PERIOD 



BLACKWELDER and Barrows, — Elements of Geology, pp. 358-368. 



Chamberlin and Salisbury, — Geology, Vol. 2, pp. 418-495. 



GRABAU and Shimer, — North American Index Fossils, Vols. I and 2. 



GsABAU, A. W., — Early Paleozoic Delta Deposits of North America: Bull. Geol. Soc. 



Aim rira, Vol. 24, No. 3. 

 ScHUCHERT, (Has., — Paleogeography of North America: Bull. Geol. Soc. America, 



Vol. 20, [910, pp. 491-493. 

 SCOTT, W. B., .hi Introduction to Geology, pp. 590-608. 

 I ik" 11. I ( ^. Revision of the Paleozoic Systems: Bull. Geol. Soc. America, Vol. 22, 



I9II. 



