i 



ii8 EVOLUTION OF THE EARTH 



at the periphery of their migratory area. This is certainly 

 true of these protochordates which, save for a few pelagic 

 forms, are all found near the continental margins in the 

 shallow seas. They are all degenerate and more or less 

 sedentary, and hence could not maintain themselves in their 

 fluviatile habitat and were swept into the sea — descendants of 

 the unsuccessful early tyros in the art of undulatory swimming. 

 But all of this requires as an impelling cause a change of 

 habitat on the part of the invertebrate progenitors, either by 

 a migration of the stock from static to dynamic waters, or by 

 a change in the character of the waters themselves. The 

 former view entails the necessity of some force to impel the 

 migration, either the need for safety or food or the lack of 

 salinity on the part of flowing waters, none of which seems 

 adequate to compel so radical a change, and one along a line 

 of so great resistance as the evolution of the invertebrate 

 ancestor into the chordate implied. On the other hand, migra- 

 tion from the sea into sluggish terrestrial waters is more 

 readily imagined, but affords no stimulus for chordate evolu- 

 tion. The picture of the Laurentian peneplain (see Fig. 17), 

 with Its low-lying lands and slow drainage, shown by glacial 

 lakes, the result of recent choking of sluggish streams, sug- 

 gests the solution, for while intermlgration under such con- 

 ditions would doubtless be relatively easy, it would never 

 have stimulated chordate evolution. Geology records a great 

 diastrophic movement toward the close of the Proterozoic, 

 the so-called Grand Canyon revolution, partial evidence for 

 which may be seen In the 8,000 to 12,000 feet of sediments, 

 more or less conglomeratic, which were swept from mountains 

 to the eastward and accumulated In the southern Appalachian 

 region during the early part of Lower Cambrian time. This 

 great upheaval changed the face of nature In many regions 

 and quickened the static terrestrial waters to rapid and wide- 

 spread movement over all the uplifted lands. Such inverte- 



