INTRODUCTION. 



The author has been led to attempt this somewhat elaborate discussion 

 of the environment of life in the latter part of Paleozoic time by many 

 considerations. Two, however, have been dominant: 



(i) He has for many years been concerned with a study of the vertebrate 

 life of the period here discussed and has, as a preliminary to more extensive 

 work, recorded the results of his work on the morphology and surroundings 

 of that form of life in the publications of the Carnegie Institution of Wash- 

 ington and elsewhere. This work, dealing more fully with the conditions 

 under which certain groups had their inception, development, and decadence 

 is a direct outcome of the previous work, and, it is hoped, the first of a series 

 which shall embrace a discussion of this period of time wherever its records 

 occur. 



(2) As the author has carried his work to the broader questions which 

 have developed he has become increasingly aware of the lack of agreement 

 as to the content of paleogeography and the method of procedure in solving 

 the various problems which arise. This publication has become, in conse- 

 quence, in part an attempt to crystallize in some measure our ideas of the 

 meaning and methods of paleogeography. The chapter upon the elements 

 of a paleogeographic problem is an effort to set forth in orderly form the 

 principles upon which such work should be concluded and the matter 

 that should be treated. It is the result of the direct question which the 

 author put to himself: What are the things that I should keep in mind 

 when attacking a paleogeographical problem in the field? The size to 

 which the answer speedily grew was somewhat appalling, but served as a 

 very vivid illustration of the need for just such an analysis, primarily for 

 the guidance of workers in the beginning of their labors. To such workers 

 the chapter on the elements of a paleogeographic problem is especially 

 addressed. 



It is perhaps needless to state that to the author the term paleogeography 

 involves a far more complex concept than is usually recognized by writers 

 upon zoogeographical or paleogeographical (sic) subjects. For the discus- 

 sion of the content of paleogeography the reader is referred to the chapter 

 on the elements of a paleogeographical problem; it may be permitted to 

 quote here a paragraph from that discussion : 



"Paleogeography is the geography of past time and is far wider in its scope 

 than a mere record of the extent of a bed or a formation, or the distribution of 

 animals or plants in any period of time. It involves all the factors which must 

 be considered in a modern geographical study, except the economic features appli- 



