AMPHIBIA 



175 



single mutation. The rest of the change would be one of gradual func- 

 tional adjustment. It was noted that the present-day "lobe-fins" 

 use the pectoral fin like a land limb in that they support the weight 

 upon it while resting on the bottom; so a fimc- 

 tional change may readily have preceded the 

 radical structural change. This theory of the 

 origin of the amphibian pentadactyl limb is 

 well shown in Fig. 102, A, B. 



Palaeographers inform us that the climatic 

 conditions of the Upper Devonian were such 

 as to encourage the development of land life 

 on the part of fishes hving in inland waters. 

 There were periods of warmth and heavy 

 rainfall followed by long periods of drought, 

 which became progressively more prolonged. 

 Such conditions would tend to drive a large 

 proportion of the non-air-breathing fishes from 

 the fresh waters and to give their place to the 

 air-breathing crossopterygians and their kin. 

 With increasingly prolonged dry seasons the 

 sestivating habits of the early lung-breathing 

 fishes proved inadequate and it became neces- 

 sary for the animals to live an active fife in the 

 air and to get their food on the land. It is probable that although 

 many early lung-breathing fishes made the beginnings of adaptation 

 to true land life, only one type fully succeeded and became the first 

 true Amphibia, the ancestors of all of those to-day living. 



Adaptive Changes Incident to Life on the Land. — ^The change 

 from aquatic to terrestrial fife has been the greatest evolutionary 

 crisis in vertebrate history. No other environmental change pos- 

 sible for animals requires so radical an alteration of developmental 

 and nutritional (in the broadest sense) mechanisms. Changes 

 from salt to fresh water, from shallows to abysses, from surface 

 to subterranean, arboreal or aereal life, involve much less funda- 

 mental alterations than does that from water to land; which is, 

 strictly speaking, rather a change from water to air. Naturally the 

 most important changes had to do with respiration, circulation, and 

 locomotion. Changes of secondary value concern the altered specific 

 gravity, the more pronoimced changes in temperature, the tendency 



Fig. 101. — Earliest 

 known fossil foot-print, 

 Thinopus antiquus, with 

 two fully formed digits, 

 I and II, a budding third, 

 III, and a possible rudi- 

 ment of a fourth, IV. 

 Upper Devonian of Penn- 

 sylvania. }/2, natural size. 

 (From Lull.) 



