AND ITS INHABITANTS 119 



brate stocks as could neither cling to the bottom nor stem the 

 quickening current were swept to the encircling sea and lost 

 to the limnobiotic fauna ; such as could, remained to people the 

 fluviatile realm with creatures of a markedly higher sort. 



The first recorded chordate comes from the Ordovician, but 

 is an armored fish-like form, an ostracoderm, of grovelling 

 habit and implied quiescent habitat (see Fig. 16, C). Such 

 types, Patten 3 to the contrary, can hardly be considered as the 

 first expression of chordate evolution, much less annectant 

 forms between the vertebrates and the higher arthropods 

 which they so closely resemble ; but they were rather a highly 

 specialized offshoot of the primitive chordate stem, derived 

 after the physical stimulus of quickened drainage had spent its 

 force and times of quiescent waters had again appeared. 



The evidence points, therefore, to Chamberlin's conclusion, 

 that the place of chordate origin was the flowing land waters, 

 to which may be added as the impelling cause a diastrophic 

 movement which quickened the drainage, and, as the time of 

 its inception, the Epi-Proterozoic interval. This crisis is there- 

 fore the direct outcome of earth movement without the inter- 

 vention of the climatic factor. 



Emergence of terrestrial vertebrates. The waters, while 

 a very necessary stimulus to early chordate evolution, afford 

 too restricted an environment for the evolution of higher 

 forms, and as a consequence all vertebrates whose ancestry 

 can be traced back through an unending line of water inhab- 

 itants since the beginnings of life on earth are but fishes, and 

 no matter to what degree they may have been specialized, 

 they could not have risen nor can they ever rise to a higher 

 plane. The emergence from the limiting waters to the limitless 

 air was absolutely essential to further development and consti- 

 tuted one of the greatest crises in organic evolution. 



To know the route of such migration is of the utmost im- 



8 Patten, W., The evolution of the vertebrates and their kin, 1912. 



