DISTRIBUTION OF SHORE ANIMALS. 325 



the absence of plants, and the consequent limitation of the 

 food-supply, the low temperature, the high pressure, and so 

 on make it very improbable that the most primitive 

 animals lived there. The problem before us, therefore, is 

 really, Were the primitive animals littoral or pelagic 1 The 

 evidence upon which the judgment must be pronounced is 

 derived first from the geological history of animals, and 

 second from their life-history. 



What does geology teach us as to the origin and 

 antiquity of shore animals? The earliest fauna we know 

 is that of the Lower Cambrian rocks, and, especially in 

 America, numerous fossils have been obtained from these 

 beds. The fossils are, generally speaking, littoral in type, 

 and they show that even in those far-off days the main 

 classes of Invertebrates were distinctly marked off from one 

 another ; Coelentera, Echinoderma, Crustacea, Mollusca, were 

 represented then as now in the littoral waters, and their 

 representatives showed many of the characters of the littoral 

 forms of the present day. The presence of these numerous 

 littoral animals in these old rocks, coupled with the paucity 

 of pelagic forms, may seem to prove decisively the greater 

 antiquity of the former; but the apparent strength of the 

 argument is diminished by two considerations, In the 

 first place, though in those old rocks there are actually 

 imprints of jelly-fish, yet generally the animals which are 

 abundantly represented as fossils are those only which 

 were possessed of hard parts. Now, as we have already 

 seen, it is characteristic of shore animals that their hard 

 parts are well developed, while pelagic animals have usually 

 little in the way of skeleton. The abundance of fossil 

 littoral animals, even in very old rocks, may then be due 

 to the fact that these are readily fossilised, rather than to 

 their abundance relative to pelagic forms. Similarly, in 

 the second place, those old rocks were laid down not far 

 from land in relatively shallow water, so that littoral 

 forms only would be likely to become entombed in sedi- 

 ment, and so fossilised. In general, though geology shows 

 us that littoral animals are extraordinarily old, it virtually 

 tells us nothing as to their age relative to other animals. 



We are thus thrown back upon the evidence derived 

 from a study of the life-history of littoral forms, but only 



