PRESIDENTIAL ADDRESS. 15 



who regard the marine faunas as much the most reliable indices 

 of geological age. 



During geological time the open sea has certainly afforded far 

 greater uniformity of conditions for the existence and evolution of 

 the different forms of life which it has contained than the land 

 and fresh waters have done. Therefore, it is reasonable to con- 

 clude that, as a rule, the progress and ratio of the differentiation, 

 evolution, and decadence among marine forms have been more 

 uniform throughout geological, time, and over greater areas, than 

 has been the case with continental life. While, as we have seen, 

 the ratio of evolution and decadence of marine types among 

 themselves has not been so uniform as it has been assumed to 

 have been by the European paleontological standard, such a ratio 

 for the continental forms of life has often not only an extraordi- 

 nary want of uniformity among themselves, but it is often at great 

 variance with that of marine life. 



Now it seems to me that the absence of a uniform ratio of 

 evolution and decadence between marine, fresh-water, and land 

 faunas and land floras, respectively, is just what we ought to 

 expect when we consider the great variety of character of the 

 various forms of life involved, and the great diversity of physical 

 conditions under which they have existed. All that we yet know 

 of ancient continental life points to the conclusion that the evolu- 

 tion of its various forms has been subject to frequent accelerations 

 and retardations ; and that, as a rule, they have been more subject 

 to abrupt extinction than marine forms have. It is true, how- 

 ever, that some of the types among the continental faunas and 

 floras which are now living have come down to us from very 

 ancient times. It is also evident that a uniform rate of evolution 

 of similar forms of continental life did not obtain in all parts of 

 the world during the respective geological periods. An illustra- 

 tion of my meaning in this respect is afforded by our Cretaceous 

 dicotyledonous flora already inferred to. In America that flora 

 had reached the European Tertiary stage long before the close of 

 the Cretaceous period. 



