338 SCIENCE PROGRESS 



by alternations of wet and dry seasons ; and the few remaining dipnoans and 

 crossopterygians still live under these conditions. 



The earliest known amphibian skeletons were preserved in the coal-swamps of 

 the later Palaeozoic ; but their fossil record may be traced as footprints in formations 

 having the characters of semi-arid flood-plain deposits as far back as the Lower 

 Carboniferous, and thus leads back to the habitat of the river fishes. The chief 

 cause that controlled the evolution of the amphibia from fishes is found in the 

 nature of the climates of the Silurian and Devonian, which favoured those fishes 

 able to make continually larger and larger use of air during the recurrent dry 

 seasons. It is believed that the air-bladder of fishes arose as a supplemental 

 breathing organ ; and under the compulsion of seasonal dryness its use for that 

 purpose became essential. The relatively severe land conditions in which the 

 fishes found themselves in the Devonian are consequently those which determined 

 the whole course of vertebrate evolution, culminating in the appearance of man. 



The argument is far more elaborate than can be indicated in a rough sketch 

 such as this. To those who know his recent work on these subjects it will be 

 superfluous to remark Prof. Barrell's acumen in making these clearings in a 

 jungle of geological thought, to comment on his fair-minded manner, or to hold up 

 for admiration his luminous, exact, and balanced literary style. 



