526 CRITICAL PERIODS IN THE EISTOR.Y OF THE EARTH. 



periods of revolution, because they are times of rapid change, both in the 

 physical and organic world, — a change overthrowing an old and establish- 

 ing a new; order of things. They are also times of lost record. We have 

 seen that these critical periods, in comparison with the preceding and suc- 

 ceeding, are continental periods, and it is for this reason that their record is 

 usually lost. 



Now, the Quaternary is such a critical or transition period, marking the 

 boundary between two great eras. The Quaternary is also a period of 

 great and wide spread oscillations, with increase and decrease of land, — a 

 period of upheaval, erosion, down-sinking, to rise again slowly to the pres- 

 ent condition. The early Quaternary was therefore, to a marked degree a 

 continental period. Here also we have newer rocks lying un conformably 

 on the eroded edges of an older series — river sediments in old river-valleys, 

 marine sediments in fiords; in other words, we have unconfurmity on a 

 grand scale. Also, in connection with these oscillations, we have great 

 changes in physical geography, and corresponding and very wide-spread 

 chano-es in climate, and consequently corresponding rapid changes in or- 

 ganic forms. Here, then, we have all the characteristics of one of the 

 boundaries between the primary divisions of time. We have a transition 

 or critical period, — a period corresponding to one of the lost intervals ; only 

 in this instance, being so recent and being also less violent than the pre- 

 ceding ones, it is not lost. From this it follows that the study of the Qua- 

 ternary ought to furnish the key which will unlock many of the mysteries 

 which now trouble us. Some of the problems which have been or will 

 be explained by study of the Quaternary we will now briefly mention. 



I. Changes of Species not sudden. If the Quaternary were lost, and we 

 compare the Tertiary rocks with the unconformably overlying recent rocks, 

 and the Tertiary mammals with those now living, how great and appar- 

 ently sudden seems the change ! How like to a violent extermination and 

 re-creation ! But the Quaternary is fortunately not lost, and we see that 

 there has been no such wholesale extermination and re-creation, but only 

 gradual though comparatively rapid transition. 



II. Migration One Chief Cause of Change. But what is sitU more im- 

 portant, we are able to trace with something like certainty the cause of 

 these rapid changes, and we find that in the higher animals, chief among 

 these causes have been wu'«?m^/ons,— migrations enforced by changes of cli- 

 mate, and migrations permitted by changes of physical geography opening 

 gateways between regions previously separated by impassable barriers. 

 This point is so important that we must dwell upon it. Only an outline, 

 however, of some of these migrations and their eifects on evolution can be 

 given in the present condition of knowledge. 



During Miocene times, as is well known, evergreens, allied to those now 

 inhabiting Southern Europe, covered the whole of Europe as far north as 

 Lapland and Spitzbergen. In America, Magnolias, Taxodiums, Libocedrus, 

 and Sequoias very similar to, if not identical with, those now living on the 



