FAUNAL RELATIONS OF EARLY VERTEBRATES 175 



with North American forms. We have yet much to learn about the 

 Mesozoic fauna of South America, but, so far as our knowledge yet 

 goes, there is a close relationship between them. This similarity, 

 of course, may have been the result of a westward migration from 

 Africa to South America by the way of a southern land communication, 

 and a concurrent intermigration of the same types from Africa north- 

 ward to Europe and thence by the north Atlantic to North x'Vmerica. 

 But a simpler explanation would be that of a land communication 

 between North and South America, and a single trans- Atlantic bridge, 

 which, in my opinion, was the southern one. 



It is very true that such hypotheses as I have offered are largely 

 based upon negative evidence. Future discoveries may bring to 

 light, both in Europe and America, types which now appear to have 

 a more restricted geographical distribution; especially may future 

 discoveries in South America and Africa show more distinctive types, 

 or, on the other hand, more common forms. I do believe, however, 

 that the long-continued exploitation of the Mesozoic rocks of North 

 America is gradually converting negative into positive evidence; 

 that we may say with tolerable certainty that certain types of land 

 vertebrates, such as the Proterosauria, Proganosauria, Pareiosauria, 

 Therodontia, etc., have never existed in North America. 



In the accompanying table I have given, as fully and as accurately 

 as the present state of our knowledge will permit, the geological range 

 and distribution of the larger groups of air-breathing vertebrates, 

 with especial reference to North America. In not a few instances 

 precise stratigraphical data are wanting, so that groups must be 

 recorded throughout a division of the chart, which later may be found 

 to have a more restricted range. An attempt has been made to indi- 

 cate by association the phylogenetic relationships of the groups, but 

 it must be remembered that opinions differ not a little concerning the 

 phylogeny of the reptiles, and those expressed in this chart are merely 

 the ones which seem most reasonable to myself. I am indebted to 

 Dr. V. Huene for a number of suggestions and facts of distribution 

 which have been incorporated in the table; and to Dr. W. D. Matthew 

 I am also obliged for the data for the mammals. It is to be hoped 

 that Dr. Matthew will confer a favor upon us all by publishing soon 

 a complete table of the distribution and range of the mammals; no 

 one is more competent. 



