134 GEOGEAPHICAL DISTKIBUTION. 



zone of life, which, as dependent upon an excess or deficiency in 

 illumination, probably does not extend much beyond a depth of 

 fifty fathoms. 



There can be but little doubt that a pelagic fauna antedated all 

 the faunas of the globe, and that from it, through a long process 

 of modification and adaptation, have been derived the faunas of 

 the shore, the abyssal deep, the land-surface, and the various fresh- 

 -waters. The identity, or close resemblance, existing between the 

 larval forms of many of the most divergent animal groups clearly 

 indicates the lines along which modification has resulted, for it can 

 scarcely be conceived, as Professor Moseley well insists, that this 

 general identity in larval structure could have been brought about 

 as the result of natural selection after the adult forms had largely 

 diverged from one another. The earliest traces of a pelsJgic fauna 

 are indicated in the rocks of Cambrian age, where, as representative 

 of it, we find, besides the remains of pteropods, the impressions of 

 jelly-fishes, which were apparently not very far removed from some 

 modern Scyphomedusae. The marine animals that are deficient or 

 lacking in the composition of the pelagic fauna, and not improbably 

 have always been lacking, are the sponges, alcyonarian corals, 

 sipunculoid worms, brachiopods, lamellibranchs, and echinoderms. 

 The true infusorians (Ciliata) appear to be but very feebly repre- 

 sented, although there is an abundance of the Cilioflagellata. 



Nature of the Littoral Fauna.— That the littoral fauna is 

 either wholly or in great part a derivative of the free oceanic or 

 pelagic fauna there is every reason to believe. The supposition 

 that the latter came into existence before the former is at once a 

 natural one, and is supported, apart from general zoogeological 

 considerations, by the character of the mutually related littoral 

 larvse, whose adaptation to a pelagic existence clearly indicates the 

 nature of their primal condition. There is, further, every reason 

 to believe that the earliest plants were also largely pelagic, and 

 that not until these had firmly established themselves as permanent 

 forms along the sea-border was there developed a shore-fauna. In 

 exposing themselves to the manifold conditions, such as the break- 

 ing of the surf, tidal action, shore-wash, attacks of enemies, 

 which a change of abode entailed upon the members of the pelagic 

 fauna, these were by force of adaptation compelled to undergo par- 

 ticular modifications of habit and structure which rendered them 



