Geographic and Geologic Relations 403 



Australia, it implies that all of these geographic regions 

 were at some time, and to some extent, in continuity. 

 Against such a conclusion it may be objected that detached 

 and floating fragments of land may have drifted across 

 oceanic expanses, and reaching other land masses, carried 

 new and important plants as well as animals to the latter. 

 The history of freshwater fishes entirely militates against 

 such a possibility. But any one who will make a detailed 

 study of some one large plant family of evolved type, and 

 somewhat recent appearance on the earth, must be gradually 

 convinced that continuous extension, even it may be with 

 gradually changing configuration, in definite areas of the 

 earth, alone give a key to the situation though such may 

 seem at first to be against preconceived views. 



Now in past monographs on fishes, each monographer 

 candidly confessed that a natural key to the evolutionary 

 distribution of fishes has not been secured. In 19 12 and 

 again on succeeding occasions up to 19 17, the writer pub- 

 lished his view, that a successful explanation could alone 

 be got if it were accepted that fishes evolved as inhabitants 

 of freshwater areas, and in some groups only gradually 

 spread at varying periods into the sea. Such is the main 

 thesis of this volume. To prove the truth of this one must 

 take account of the relation of land and sea, from time to 

 time at least, when fishes first appeared as fossils in the 

 rocks. Such in turn involves combined geographic and geo- 

 logic studies, with all accompanying details of each of these 

 studies. 



If in a few succeeding pages the writer discusses some 

 geologic positions first, this may later conduce to clearer 

 geographic views. In chapters II, V and VI frequent 

 reference was made to the wholesale destruction of myriads 

 of plant and animal types, at definite stages in geologic 

 periods. But equally noteworthy has been the extrusion, 

 between or upon rock masses already formed, of enormous 

 deposits of igneous rock that often cover many thousands 

 of square miles. A third and equally noteworthy event 

 has been the tremendous faulting as well as folding and 

 even overthrowing of hundreds — in some cases of thou- 

 sands — of miles of strata, due in part probably to internal 



