PREFACE. V 



fluvial, in the narrower sense, but embrace also the products of gen- 

 eral aggradation on surfaces of low gradient, by pluvial and other 

 subaerial agencies. 



On the physical side, the chief effort has been to give a connected 

 and interpretative sketch of the earth's history, especially as exem- 

 plified by the North American continent. Imperfection of knowledge 

 makes this sketch incomplete at some points, and limitations of space 

 have made it brief, even when knowledge is more adequate. Both 

 imperfection of knowledge and limitations of space have made it imprac- 

 ticable to deal with other continents as fully as with our own. It 

 is believed, however, that the principles set forth in connection with 

 the history of our own continent are applicable to all continents, 

 and that the course of events in all has been, in principle and in lead- 

 ing facts, similar to that in our own. 



While recognizing the alternative views, the doctrine of the per- 

 manence of the continents plays an unusually large part in the inter- 

 pretations of continental evolution, of the migrations of life, and of 

 the successive developments of provincial and cosmopolitan faunas. 

 So also the doctrine of the periodicity of the great deformative move- 

 ments forms a notable feature in the interpretations of life evolution, 

 and in the reciprocal developments of land and sea life, as well as of 

 provincial and cosmopolitan faunas. 



Exceptional attention is given to the evolutions of faunas and 

 floras and to their radiations and migrations, while less attention is 

 given to the unrelated features of the life, however bizarre these may 

 be in themselves. The familiar calling of the biological roll under 

 each period is abandoned, and will perhaps be missed; but it has been 

 thought that the mutations of the assemblages of composite life of 

 the great provinces that arose in succession from changes in geographic 

 configuration, are more important as elements in the history of the 

 earth's inhabitants than the classification of life forms as such, par- 

 ticularly as such classification is still in a state of transition. The 

 order of treatment of the various forms of life in the several periods 

 is varied according to what is conceived to be their historical impor- 

 tance or natural relations. 



Without giving special adhesion to any particular doctrine of life 

 development, beyond a cordial recognition of the new mutation theory 

 of DeVries, as well as the older and more familiar ones, the progressive 



