280 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



velopment of the visual organ is partly a matter of growth and decline 

 and in a large part an adaptation. The pigment about the ommatidia 

 of a compound eye can by its readjustment adapt the organ either to the 

 dusk or the brightness, and an elaborate compound eye thus fits all 

 depths, shallow or great. I suggested last year that conditions of long 

 isolation are sometimes, more often than we think, indicated by the re- 

 sumption of primitive ornaments in the trilobite integument at a stage 

 where in the normal progress of the race the creatures have burst into 

 their climax of decoration, as in the trilobites of the austral Devonic, 

 where even the sutural spines of the Cambric reappear in conjunction 

 with the climacteric decoration of the race appropriate to the Devonic 

 period. Here, too, isolation shows its effects in the development of ex- 

 pressions of lobal coalescence and pygidial decoration not elsewhere 

 known. These characters in morphology are positive factors in the deter- 

 mination of the limitations of the strand, and morphology does in its 

 total expression prove the most dependable index of geographic differ- 

 ences. The eurypterids, in their life history and in their climax, are far 

 more sensitive to geographic changes. The few early eurypterids we 

 know were doubtless marine, and the creatures gradually acquired the 

 brackish-water habit of their climax, which seems to have eventually 

 changed to a fresh-water life. The value of these creatures as indexes of 

 geography at any one time in the earth's history is therefore quite evident. 



