802 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 
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 débris of the littoral and terrestrial faunas and floras, became 
abundant in deep water.” 
‘*Tt 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~fresh-water and terrestrial 
faunas. 
(¢) 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. 
Fresh Water 
Shore ay 
Open Sea 
