760 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



lizards began to swoop from branch to branch ; some of the 

 ancient Saurians flopped 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." 



The Relation of the different Faunas to one another. 



As we have already hinted, the problem of the evolution 

 of faunas is still beyond solution, and as this is not the 

 place for the marshalling of arguments, I shall content 

 myself with stating various possibilities. 



(a.) According to Moseley, "The fauna of the coast has not only 

 given origin to the terrestrial and fresh-water faunas, it has throughout 

 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 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." 



(b. ) 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 fresh-water and terrestrial 

 faunas. 



(e.) 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. 



