406 T. C. CHAMBERLIN 



chemical nature of all the Paleozoic and most of the Proterozoic 

 strata, afford, in my judgment, ample ground for the presump- 

 tion that vegetation clothed the land from a date long anterior 

 to the Paleozoic era and that the land waters were capable of 

 supporting their own appropriate fauna, as well as contributing 

 to the support of that of the sea border. 



Now there is one distinctive characteristic of land waters 

 that deserves consideration in the study of the evolution of the 

 early vertebrates, because it was a strenuous dynamic condition 

 constantly impressed upon their fauna. It is their most familiar 

 and essential feature, their flow. Neglecting lakes, which are 

 mere incidents, land waters are distinguished by persistent and 

 usually rather rapid motion in a fixed direction, and this is an 

 insistent physical condition to which their fauna must adapt itself. 

 Fortunately this adaptation must take a tangible form, whereas 

 adaptation to the freshness of the water is accomplished by 

 obscure modifications which are not as yet detectable. In flowing 

 water, the animal must maintain its position against the current 

 either by a contact of some resisting kind with the bottom of the 

 stream, or must be provided with an effective mode of propulsion 

 competent to meet the constant force of the current without 

 undue draft on the vital resources; otherwise the animal would 

 be swept out to sea and its race be ended as a stream-dweller. 

 It is different with ocean currents, for they return upon them- 

 selves and an animal may yield to them without losing its marine 

 habitat; and besides, they are usually much feebler than river 

 currents. 



A glance at the faunas of existing streams, which represent 

 the outcome of ages of trial, shows only three prominent groups 

 of animals that have accomplished the adaptation. The minor 

 instances are negligible. The successful cases are, first and fore- 

 most, fish, second, certain mollusks that crawl on the bottom 

 with firm contact, and third, certain crustaceans that are provided 

 with numerous sharp claws that give them ready catch and hold 

 upon the stream bed. The brachiopods that are free in youth, 

 but sessile or pediceled in later life, the cephalopods that are 



