96 LIFE : OUTLINES OF GENERAL BIOLOGY 



branch; some of the ancient Saurians flapped their leathery skin- 

 wings; a few arboreal mammals essayed what the bats perfected; 

 and the feverish birds flew aloft gladly. 



Perhaps a keen struggle among insects, or such events as floods, 

 storms, and lava-flows would prompt to flight; perhaps it was the 

 eager males who led the way; perhaps the additional respiratory 

 efficiency, produced by the outgrowth of wings, gave these a new 

 use. Perhaps the high temperature of birds — an index to the intensity 

 of their metabolism — may have had to do with the development of 

 those most elaborate epidermic growths which we call feathers. 

 But we must still be resigned to a more or less ingenious "perhaps". 



EVOLUTION OF FAUNAS.— The problem of the evolution of 

 faunas is still beyond solution, but various possibilities may be stated. 



{a) According to Moseley, "the fauna of the coast has not only 

 given origin to the terrestrial and freshwater faunas, it has through- 

 out all time, since life originated, given additions to the Pelagic 

 fauna in return for having received from it its starting-point. It has 

 also received some of these Pelagic forms back again to assume a 

 fresh littoral existence. The terrestrial fauna has returned some forms 

 to the shores, such as certain shore birds, seals, and the polar bear; 

 and some of these, such as the whales and a small oceanic insect, 

 Halobates, have returned thence to Pelagic life. 



"The deep sea fauna has probably been formed almost entirely 

 from the littoral, not in the most remote antiquity, but only after 

 food, derived from the debris of the littoral and terrestrial faunas 

 and floras, became abundant in deep water. 



"It was in the littoral region that all the primary branches of the 

 zoological family-tree were formed ; all terrestrial and deep-sea 

 forms have passed through a littoral phase, and amongst the 

 representatives of the littoral fauna the recapitulative history, 

 in the form of series of larval conditions, is most completely 

 retained." 



(6) According to Agassiz, Simroth, and others, if one may venture 

 to compress their views into a sentence, a littoral fauna was the 

 original one, whence have been derived, on the one hand, the Pelagic 

 and abyssal faunas; on the other hand, the freshwater and terrestrial 

 faunas. 



(c) According to Brooks, a Pelagic fauna was primitive, whence 

 have been derived the tenants of the shore and the inhabitants of 

 the deep sea. To the latter, however, a possibility of ascending again 

 is not denied. 



(d) Sir John Murray has emphasised the importance of "the 

 mudhne"- -the lower boundary of the littoral area — as an important 

 headquarters of animal life, and as the area from which the abysses 

 were peopled. The possibilities have been expressed in Fig. 19. 



