The geographical distribution of the terrestrial plants and animals is undoubtedly much easier to be ascertained than that of the denizens of the ocean. The naturalist is able to climb the highest mountains beyond the extreme limit of vegetation, and far above their most towering peaks his eye, piercing the transparent atmosphere, sees the condor soar in solitary majesty; he can wander through the deepest glens, or even, penetrating into the bowels of the earth, examine and collect the forms of the subterranean flora; but it has not been given him to perambulate the submarine meads, or to force his way leisurely through dense thickets of algæ, and explore their hidden wonders.
Yet, in spite of these natural impediments, his inventive genius, fired by his insatiable avidity of knowledge, has given him the means of interrogating the abyss, and partly raising the veil behind which marine life conceals its secret operations. Armed with a dredge, he fetches from the bottom of the sea plants, polypi, mollusks, and annelides, and learns to distinguish the various depths assigned for their abode; or he puts on the helmet of the submarine diver, and passes whole hours in collecting and observing beneath the clear waters of the sea; or he drops the plummet hundreds of fathoms deep into the ocean, and draws it up again coated with specimens of corals or Foraminifera.
To the late Professor Edward Forbes of Edinburgh science is indebted for the first investigations of this nature that have been undertaken on a greater scale; and, to give the reader some idea of the causes which regulate the distribution of marine life, I cannot do better than cite a few of the general results of that eminent naturalist's researches.[U]
[U] Natural History of the European Seas, by the late Professor E. Forbes. Edited by R. Godwin Austen, 1859.
As the animals and plants of the land are grouped together into distinct zoological and botanical provinces, so likewise is the population of the sea gathered into geographical groups, which, though well marked in their more central and most developed portions, imperceptibly merge at their margins into those of neighbouring realms. "These submarine provinces have a more or less direct correspondence with those of the neighbouring lands, though sometimes they differ very considerably from the latter in their extent; since the physical features which may constitute boundaries in the one, may not be sufficiently extended or developed in the other to impede the spread of peculiar species of animals or plants. Marine creatures, owing to their organisation and the transporting powers of the element in which they live, are much more capable of diffusion, as a whole, than the terrestrial organisms; hence we should expect to find the regions they respectively inhabit, beneath the waves, of much vaster dimensions than those occupied by similar geographical assemblages of their terrestrial brethren; and such is to a great extent true. Nevertheless, the inequalities of the sea-bed, the modifications of the temperature of the ocean produced by currents pouring through it like mighty rivers, the projection of promontories, and the more important interruptions caused by the great gulfs and abysses of the deep, or by vast and comparatively desert tracts of unprolific sand, which in many places are spread out in extensive shallows, are all-powerful influences, determining their diffusion within certain and more or less defined limits."
The structure of the coast, as far as the mineral character of its rocks is concerned, may seriously affect the distribution of particular tribes. Since many shell-fish, for instance, bore only in limestone or rocks containing abundance of lime, a very ordinary difference in the nature of the strata must necessarily determine their presence or absence.
The outline of a coast has also great influence in regulating the diffusion of species. A much indented region is very favourable to submarine life; a straight coast-line, exposed to the full rolling of the surf, is usually unfavourable, though there are a few creatures which delight in the dash of the waves, and hardily, though some of them are small and exceedingly delicate, brave the full force of the ocean storms, reminding us, as Mr. Godwin Austen quaintly remarks, "of those sturdy people, not uncommon in this stormy life, who thrive best in troubles, and feel happiest under conditions that make most men miserable."
The nature of the sea-bottom, according as it consists of mud, sand, gravel, nullipore, broken shells, loose stones, or rock, determines, to a great extent, the presence or absence of peculiar forms of shell-fish and other invertebrata, and of fish also, since the distribution of the food regulates that of the devourers.
The rise and fall of the tides are most important in determining the presence or absence of the species inhabiting the littoral zone. The currents, besides their agency as modifiers of climate, act as means of transport, by carrying the germs and larvæ of numerous creatures from region to region.
The influence of climate is conspicuously manifested in the diminution of the number of genera and species as we proceed northwards to the Icy Ocean.