764 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



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. 



(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 mud- 

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

 quarters of animal life, and as the area from which the abysses were 

 peopled. The possibilities may be expressed in a diagram. 



Dry Lane! 



Shore 



Original 

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