Isthmus Barriers Separating Fish Faunas 281 



Further, it goes without saying that we have no knowledge 

 of the period of time necessary to work specific changes in a 

 body of species isolated in an alien sea. Nor have we any 

 data as to the effect on a given fish fauna of the infiltration 

 of many species and genera belonging to another. All such 

 forces and results must be matters of inference. 



The present writer does not wish to deny that great changes 

 have taken place in the outlines of continents in relatively 

 recent times. He would, however, insist that the theory of 

 such changes must be confirmed by geological evidence, and 

 evidence from groups other than fishes, and that likeness in 

 separated fish faunas may not be conclusive. 



Fig. 184. — Drawing the net on the beach of Hilo, Hawaii. Photograph by 



Henry W. Henshaw. 



