Le Conte.] 



Critical Periods. 



3^5 



fore bring us no nearer the origin of the Tertiary mammals than 

 before. Osborn, however, finds them distinctly transitional, i. e,, as 

 nearly allied to the Puerco, lowest Tertiary, Eutheria, as they are 

 to the Jurassic Metatheria. 



In this revolution the geographical changes were probably much 

 greater than in the glacial, although far less known. The life- 

 changes were also much greater, except for the effect of man in the 

 latter. As the drift represents deposits during the Glacial, so the 

 Laramie those during this revolution. The Cretaceous proper was 

 before, the Tertiary after, the Laramie during the revolution. The 

 time occupied in the change and represented by the Laramie for- 

 mation was very much longer. The break in the record was much 

 greater and more general, but yet not universal as witness the con- 

 tinuity of strata in some parts of the west. Undoubtedly Eutheria 

 or true mammals appeared first in Arctogaea, as this was the great 

 field of extensive migrations and the battle ground of species, and 

 therefore the place of most rapid evolution. From thence they 

 spread by migration everywhere except isolated Australia. 



These two periods, viz., the post-Cretaceous and the post-Tertiary ; 

 were undoubtedly periods of very great and widespread changes, 

 probably effecting nearly, if not quite, the whole surface of the 

 earth. Now, have we anything at all comparable to these in the 

 intervening Tertiary? — Surely not. They are therefore properly 

 called revolutionary periods, and should be made the lines of demar- 

 kation separating primary divisions of time. 



Among the changes of climate, perhaps of many kinds which 

 took place in the last or glacial revolution, we find the most con- 

 spicuous is that of extreme cold. Do we find any such in the post- 

 Cretaceous revolution? According to Dana there are some evi- 

 dences of a cooler climate than before or after. But the evidence is 

 not clear, nor is it probable that the lowering of temperature was 

 extreme. 



III. THE POST-PAL.EOZOIC OK APPALACHIAN REVOLUTION. 



Going still back the next, and perhaps the most conspicuous of 

 all, is that which separates the Palaeozoic from the Mesozoic. If 

 we call the Permian a transition or critical period, corresponding to 



