410 T. C. CHAMBERLIN 



inevitably in the changing relations of land and sea that attend 

 the normal progress of geologic periods. Thus, on the head- 

 waters by piracy and along the sea by lagoons, there are syste- 

 matic sources of intercommunication by which fresh-water 

 faunas may migrate from basin to basin and may thus occupy 

 quite fully their appropriate domain without dependence upon 

 accidental means or coast-wise communication by temporary 

 entrance of the sea, which may be a resource in some cases. 

 Measured by geologic periods, these means of migration are 

 doubtless sufficiently frequent to be altogether adequate. 



The extensions and the changes of domain thus provided to 

 the hypothetical primitive chordate organism may be assumed 

 to have sufficed for its expansion and differentiation through a 

 long period without giving rise to any such degree of overcrowd- 

 ing as to force it to take to the sea for relief ; and such intru- 

 sions upon the sea as occurred during this initial era may be 

 regarded as individual and accidental. 1 If this be so, it may be 

 inferred that even after the primitive chordates had become dif- 

 ferentiated into the ancestral classes, and even into the main 

 ordinal types of the fishes as we now know them paleontologi- 

 cally, and had also attained some measure of induration of parts, 

 the preservation of these parts in the sea sediments would be 

 rare ; and this is in accord with experience. 



In time, however, the streams must inevitably have become 

 overstocked and a severe struggle for existence must have 

 ensued attended by the acquisition of organs of attack and 

 defense ; and at length there must have been an irruption into 

 the sea to avoid the greater enemies and the stronger competi- 

 tion at home. To such an irruption is assigned the remarkable 

 apparition of agnathous and gnathous fishes at the close of the 

 Silurian in varied type and clothed in full armor, expressing the 

 urgency of the competition ; though a notable part of the 



1 From early individuals that failed to hold their place in the streams for any 

 •reason, and succeeded in maintaining themselves in the sea to which they were car- 

 ried, there may have sprung the lower chordate types like the ascidians and Balano- 

 ^glossus, if they really belong to the vertebrate phylum. 



