i4r)8 
THK VOYAGE OF H.M.S. CHALLENGER. 
salt as many regions of the present ocean, that they had a relatively high temperature 
and were probably frequently laden with fine clayey matter derived from the land and 
shallow reaches of the sea-bed, a condition of matters that now to some extent obtains in 
the western Pacific Ocean. ^ The great abundance. of Protistan life in the open waters of 
the early ocean of which we have here some evidence, produced, in all likelihood, by 
death and decay a most abundant supply of food at the bottom of the ocean, especially 
just beyond the mud-line. In this position, again, the abundant food supply would 
brinjr about the most favouralde conditions for the evolution of the Metazoa. Some 
.species would eventually be able to spread upwards and establish themselves in the 
shallower portions of the sea-bed swept clear of mud by tides and currents. Even in 
these early ages species with pelagic larvm, from being able to disperse more rapidly 
and to occupy all points of vantage, would ultimately be the successful competitors 
in these shallow reaches of the ocean.” 
In the tropical and sub-tropical regions of the present ocean we find many large 
overgrown larval forms, — Plagusia, Leptocephalus, Cephalacanthus, Phyllosoma, Alima, 
Erichthus, Arachnactis, &c. — derived from the shore and shallow- water Benthos. These, 
having been carried by currents into the open ocean, and, not finding there the bottom 
conditions favourable for their further development, have grown to an enormous size. 
In some cases larvae of this kind may have acquired the power of reproduction in their 
new habitat, somewhat after the manner of the Axolotl. Again, it is not difficult to 
imagine the pelagic Coelenterates to have been derived from some such form as a 
Medusoid gonophore which, on being carried into pelagic waters, could not find the con- 
L.vrv.*;. ditions necessary for the development of the fixed generation.^ My late colleague, 
Professor Moseley, used to regard the pelagic animals as the original stock from which 
other marine faunas had sprung,^ but evidence from embryology and other departments 
of research shows that there are better reasons for holding that all the highly organised 
pelagic animals are descended from ancestors which lived in the shallow waters 
suiTounding the land, while the very ancient pelagic Protophyta and Protozoa were 
probably derived at a much earlier date from the simplest forms of life that originally 
appeared at the mud-line of Pre-Cambrian seas. The mass of individuals in the pelagic 
fauna and flora probably greatly exceeds that of other marine faunas, still the species are 
few when com})ared with the organisms of the shore and shallow water. Whales, seals, 
pelagic Fishes, JIalohuteis, pelagic Ceplialojiods, lanthina, Scylluca, Pteropods, Heteropods, 
pelagic Cnistaceans, Worms, and Coelenterates, all appear to bear distinct traces of a 
' See liuchan, Report on Oceanic Circulation, Phys. Chem. Ghall. Exp., jit. viii., pp. 3, 13, ISDS. 
* See W. K. Brtwkn, The Genus Sal pa, Johns Hopkins Press, Baltimore, 1893, p. 123 ; also The Origin of the 
(»l>lcst Fossils anfl the Discovery of the Bottom of the Ocean, Journal of Orology, vol. ii. p. 455, Chicago, 1894. 
* See Agassiz, Three Cruises of the Blake, vol. i. p. 208. 
* See 7/ViV. Au. Hrptni for 1884, i>j». 751-753. 
