In general, the dominant groups of land animals are the mam mals and insects, while the dominant groups of marine animals are the Crustacea and fishes. The dominant land plants are, the flowering ones while the dominant marine forms are the Diatoms and the bottom-living Algae. By "dominance" we mean abun dance, ubiquity of distribution and variety of adaptations. There are few places on the land where one does not find mammals, while there is no region of the sea where there are not fishes. Similarly, insects of some kind are distributed everywhere, almost, on the land, in the soil, in the air, or even parasitic in other ani mals or in plants; just the same can be said of the occurrence of the Crustacea in the sea.
Sea weeds, zoophytes, barnacles, worms and many molluscs are typical inhabitants of this zone. Below the level of the low water marks of spring tides is the Lamznarian zone, so-called because of the characteristic abundance of the large "tangle," or Latni naria. We may take this region to extend out to sea as far as the depth of io fathoms (though Darwin found Laminaria in Patagonian seas, growing up to the surface from a depth of about 45 fathoms). Outside this region is the Coralline zone in which the ordinary red, green and brown algae begin to disappear and where the calcareous algae, the Nullipores, are abundant. (Forbes im agined this region to be bounded by the limiting depth of 300 fathoms.) In deeper water he regarded the bottom as being life less: at least if life existed it exhibited "but a few sparks to mark its lingering presence." Outside the 30o fathom depth was, in his conception, the "Azoic zone." Nowadays the Littoral, Laminarian and Coralline zones of Forbes still retain a general validity. But we should say that characteristic faunas begin to be recognized when the depth ex ceeds about ioo fathoms. We add to Forbes's categories, there fore, a Deep Water zone and an "Abyssal zone." It is impossible to delimit these regions except in a very general way but we may think about the former as contained between the i oo and the I,000 fathoms contour lines and of the latter as being the im mense region of sea bottom where the ocean is more than about I,000 fathoms in depth.
Thus no part of the ocean is lifeless so far as our investi gations go. The conditions vary remarkably: in the greatest abysses there is absolute darkness, a temperature which is just about freezing point, and pressures that are measured by tons to the square inch—yet living fishes and invertebrates are there. In polar seas beneath the ice there may be temperatures that are lower than that of the freezing point of fresh water, yet some form of life may be extraordinarily abundant. Rather high tempera tures (up to 3o°C. or 86°F.) are found in the Red sea, yet life is also abundant there. Even in the stagnant and apparently poi sonous water of the lower levels of the Black sea there is plenty of unicellular life. Thus organisms have adapted their activities to almost every kind of physical conditions that is exhibited in naturally occurring water masses.