710 EDWIN C. ECKEL 



precision of such mapping is limited only by the consideration 

 of expense. Moreover, we are able in most cases to discuss 

 topographic features in terms of causality. 



With regard to the present biologic condition of the earth 

 our knowledge is much less definite. Mapping the distribution 

 of existing faunas or floras is not practicable in the present state 

 of our knowledge of biotic^ distribution, except in a very gen- 

 eral way; anything like a detailed map is impossible. This con- 

 dition is due partly to the expense of collecting sufficient data 

 on the range of the different species of plants and animals. 

 Most of the difficult}/, however, arises from the fact that the 

 principles underlying and regulating animal and plant distribu- 

 tion are still far from being well understood, though of late 

 years great advances have been made in that direction. 



If our knowledge is thus imperfect with regard to the exist- 

 ing biota, ^ our knowledge of the facts and causes of its distribu- 

 tion during past ages is still less definite. It is noteworthy that 

 two of the most successful attempts^ to account for biotic 

 distribution and evolution in periods antedating the present 

 have been the work of geologists specializing in physiographic 

 work, a branch which necessitates lithologic rather than biologic 

 discriminations. 



Obviously, the paleontologic record must always be more 

 defective than the lithologic ; for fossils are not alwa^^s found 

 where rocks are exposed. In addition to gaps occurring because 

 of local lack of fossils, two periods are particularly incapable of 

 being treated on a biologic basis ; one immensely long period at 

 the commencement of geologic history, and one short but highly 

 important period immediately preceding the beginning of written 

 history. 



With regard to the relative values of the biologic and litho- 

 logic units as measures of absolute time, the case is somewhat 



^ Biota = the sum of the fauna and flora of a region. Cf. Stejneger, American 

 Naturalist, February 190 1, p. 89. 



^ T. C. Chamberlin, "Systematic Source of Evolution of Provincial Faunas," 

 JouRN. Geol., VI, pp. 597-608, 1898. J. B. WooDWORTH, " Relation between Base- 

 leveling and Organic Evolution," American Geologist, XIV, pp. 209-235, 1894. 



