276 Joseph Barrell. 



solution of the second problem which is here especially 

 sought. ' ' 



''It is shown to be probable that fishes arose in land 

 waters. As such they constituted primarily a river 

 fauna." It is in the Middle Devonian that "the fishes 

 first really begin to conquer the ocean and its former 

 rulers." On the other hand, in the fresh waters of this 

 time fishes abounded in greater variety than in the seas. 

 In the Upper Devonian, cross opterygian fishes had risen 

 to a dominant place, and they were adapted to live in 

 warm climates marked by alternation of wet and dry 

 seasons. It was this environment that gave rise to the 

 amphibians. ''The warm and stagnant waters of the 

 dry season compelled those fishes which should survive 

 to make larger and larger use of air. ' ' 



"The evidence is regarded as strong that the air- 

 bladder was originally developed as a supplemental 

 breathing organ, although in modern fishes it has been 

 mostly diverted to other uses." Barrell also quotes this 

 significant passage from W. D. Matthew: "The evo- 

 lution of land life in adaptation to recurrent periods of 

 aridity supplies a satisfactory background of cause for 

 the whole evolution of the higher vertebrates." And he 

 adds : ' ' The rise of amphibians from river fishes in 

 an epoch of semiaridity was one of the major steps in 

 the evolution of man." "Climatic oscillation is a major 

 ulterior factor in evolution." 



The study of the natural environment as recorded in 

 the sediments that also entombed the fossils led natur- 

 ally to the work entitled "Probable Relations of Cli- 

 matic Change to the Origin of the Tertiary Ape-Man." 

 Here we again read that climatic variation is for ter- 

 restrial life its most fundamental evolutionary factor. 

 And this was especially true for the Pleistocene, when 

 the land biotas "have come and gone at the command 

 of climatic change. Those animals which were trapped 

 on the northern sides of mountain ranges or water bar- 

 riers were remorselessly exterminated by the waves of 

 advancing cold; those which could escape to the south 

 returned with milder climates, but changed in their 

 assemblage." Barrell held that man was brought to 

 his present high physical and mental state not as the 

 "mere product of time and life," but that he is 

 "peculiarly a child of the earth and is born of her 



