ENVIRONMENTAL CONDITIONS 391 



This article emphasizes this view and elaborates the evidence and its 

 bearings on the first appearance of land vertebrates. Climatic oscillation 

 is a major ulterior factor in evolution. 



Environmental Conditions surrounding the Eise of Amphibians 

 probable early history of chordates 



This paper is a study of the compulsion of changing environments on 

 the progress of evolution;, illustrated in the rise of the first land verte- 

 brates. Then was determined for all later time the general body plan,, 

 which, with its initial limitations, and yet its advantages, was to be 

 molded and adapted to all the various modes of vertebrate life. But the 

 remote ancestors of amphibians had already been in existence for geologic 

 ages before the first vertebrates emerge from obscurity and write their 

 existence plainly in the geologic record. During those previous ages the 

 environmental and organic forces were already at work, which gave rise 

 in the fullness of time to the Silurian and Devonian fishes and determined 

 a certain group of them capable of becoming ancestors of amphibians. A 

 discussion is called for, consequently, of the probable early history of 

 chordates as an introduction to the central problem of the paper. The 

 evidence is mostly indirect and is not of the same order of certainty as 

 that bearing on the rise of amphibians, but nevertheless it points some- 

 what definitely toward certain conclusions. 



The lingering and aberrant representatives of the early chordate stem 

 are now found in oceanic environments, as balanoglossus, ascidians, and 

 the lancelets. This present restriction may be thought suggestive of an 

 original marine origin. It should be distinguished, however, from proof, 

 since the actual ancestral line of fishes is not known and the greater 

 breadth and geological stability of conditions on the margins and floor 

 of the ocean would favor the longer preservation there of such archaic 

 types as had once become established in that realm. The most primitive 

 chordates, balanoglossus and amphioxus, burrow in the sea sands and live 

 somewhat after the fashion of annelids. It seems that at a time when 

 chordates may have been more abundantly of this type they should, like 

 annelids, have spread -into bottom sands and muds of both fresh and salt 

 .waters. Even if such primitive chordates had once existed in fresh 

 waters, they would have left nothing more than worm burrows, no dis- 

 tinctive fossil evidence of their existence. The ultimate place of origin 

 of chordates can not, then, be safely argued to have been the sea; much 

 less can the presence there of balanoglossus and amphioxus be held as 

 evidence of a marine origin of fishes. They leave the place of origin an 

 open question, to be decided by other lines of evidence. 



