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 func- 

 tional change may readily have preceded tha 

 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 living 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 J ^ ~ 

 proportion of the non-air-breathing fishes from Thinopus antiquus, with 

 the fresh waters and to give their place to the two fu % formed digits, 

 air-breathing crossopterygians and their kin. jjj 11 and ' ^ ^gjble ^-udi 

 With increasingly prolonged dry seasons- the merit of a fourth, IV. 

 activating habits qjf the early lung-breathing Upper Devonian of Penn- 



r i" 1 . -I -i Qvlvanm lx naf.nral aiva 



fishes proved inadequate and it became neces- 

 sary for the animals to live an active life 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 Jife 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 4he broadest sense) mechanisms ; Changes 

 from salt to fcesh 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 wafygp-4o 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 proitaunced changes in temperature, the tendency 



