MATTHIJW , CLIMATE AND EVOLUTION 173 



5. The numerous hypothetical land bridges in temperate tropical and 

 southern regions, connecting continents now separated by deep oceans, 

 which have been advocated by various authors, are improbable and un- 

 necessary to explain geographic distribution. On the contrary, the 

 known facts point distinctly to a general permanency of continental out- 

 lines during the later epochs of geologic time, provided that due allow- 

 ance be made for the known or probable gaps iu our knowledge. 



Introduction 



alternations of elevation and climate during geological time 



Several years ago,- I had the honor to give a talk upon "'Climate and 

 Evolution" before the Linnaean Society. The subject Avas then new to 

 me — it was an application to vertebrate paleontolog}' of theories in 

 regard to geological history which had been brought forward by Cham- 

 berlin a year or two previously.^ I have had these concepts more or less 

 in mind ever since, and though I must admit that I am far from having 

 the evidence in shape for final presentation, I desire to submit for gen- 

 eral consideration the conclusions thus far reached. 



Chamberlin's theories are to-day well known and are year by year 

 gaining a wider acceptance. So far as they pertain to the present sub- 

 ject, they differ from the older prevailing concept of geological climatic 

 conditions chiefly in that they involve an alternation of climates through 

 the course of geologic time from extremes of warm, moist tropical and 

 uniform, to extremes of cold, arid zonal climates. The former are the 

 results of prolonged base-level erosion and the overflow of large conti- 

 nental areas by shallow seas. The latter are the results of the re-adjust- 

 ments needed to bring the continents once more into isostatic balance, 

 involving the general lifting of the continents, especially of their borders, 

 the expansion of the continental areas to their utmost limits and the 

 renewal of rapid erosion. 



These alternations of conditions are marked by alternations of the 

 prevalent type of formation in the geological series. The uniform base- 

 leveling corresponds to widespread deposits of limestones and in its 

 waning stages Avith coal formations. The periods of uplift are marked 

 by thick barren formations, often red in color, by indications of arid 

 conditions in salt and gypsum beds and they finally culminate in great 

 extension of glaciers from boreal and high mountain areas. 



2. Tan. 14, 1902. 



^T. C. Chamberlix : .Tour. Geo!., vols, v-vlli. 1897-1901. 



