The same physical conditions are associated with a certain similarity between the animals of different provinces. Where those provinces are proximate, such likeness is due to the identity or close affinity of the species; but where the provinces are remote the resemblance is one of analogy, and species of different genera or families represent each other. A second mode of expressing the ascertained facts of the geographical distribution of marine animals is by tracts called Homoiozoic Belts, bounded by climatal lines; which are not, however, parallel with lines of latitude, but undulate in subordination to climatal influences of warm or cold oceanic currents, relations of land to water, etc. Of these belts Professor E. Forbes has defined nine: one equatorial, with four to the north and four to the south, which are mutually representative. But the most interesting form of expression of the distribution of marine life is that which parallels the perpendicular distribution of plants. Edward Forbes, availing himself of the valuable results of a systematic use of the dredge, first showed that marine animals and plants varied according to the depth at which they lived in a manner very analogous to the changes in the forms and species of vegetation observed in the ascent of a tropical mountain. He has expressed these facts by defining five bathymetrical zones, or belts of depth, which he calls: 1. Littoral; 2. Circumlittoral; 3. Median; 4. Infra-median; 5. Abyssal. The life-forms of these zones vary, of course, according to the nature of the sea bottom; and are modified by those primitive or creative laws that have caused representative species in distant localities under like physical conditions--species related by analogy. Very much remains to be observed and studied by naturalists in different parts of the globe, under the guidance of the generalizations thus sketched out, to the completion of a perfect theory. But in the progress to this, the results can not fail to be practically most valuable. A shell or a sea-weed, whose relations to depth are thus understood, may afford important information or warning to the navigator. To the geologist the distribution of marine life according to the zones of depth has given the clew to the determination of the depth of the seas in which certain formations have been deposited. By the light of these laws of geographical distribution we view with quite a new interest the shells, corals, and sea-weeds of our own shores. We trace the regions whence they have been invaded by races not aboriginally belonging to our seas; we obtain indications of irruptions of sea-currents of dates anterior to the present arrangements of land and water. Thus, part of our marine fauna has been traced back to the old Pliocene period, part to the somewhat newer period of the red-crag, part to the still more recent glacial period--all these being anterior to the constitution of the Celtic Province, as it is now displayed.