Page:Popular Science Monthly Volume 76.djvu/601

Rh greatest bulk of invertebrates are restricted to the bottom of the shallow seas within the depth to which sunlight readily penetrates, that is, a depth on the average not over 600 feet. The value of this observation to the paleogeographer and the student of fossil marine life lies in the confirmation of paleontologists that continental seas are shallow seas, to the bottom of which in most places sunlight permeates. These seas are to be compared with the littoral regions of the present oceans, and they are the areas that are most exposed to climatic and physical changes, due to their proximity to the atmosphere and the lands. The life of these waters is, therefore, subject to an environment that is more or less changeable, and one of the basic causes underlying organic change. It is the invertebrates of the littoral and shallow seas that the paleontologist studies.

In the tropical and subtropical shallow seas one meets with the greatest variety of life and with the brighter colored and more ornamental shelled animals, but we are much surprised when told that the greatest number of individuals occur in the colder shallow waters of the temperate and polar regions. Johnstone states, "There is little doubt that the distribution of life in the sea is exactly opposite to that on the land. The greatest fisheries are those of the temperate and arctic seas. . . . Nowhere are sea birds so numerous as in polar waters. The benthic fauna and flora are also most luxuriant." The Bay of Naples has a "richly varied, but (in mass) a scanty fauna and flora," and "at the very least the amount of life in polar seas is not less than in the tropics."

Marine life is also more prolific near river mouths of the temperate zones, probably because of the great quantities of dissolved "salts of nitrous and nitric acid and ammonia, and other substances which are the ultimate food-stuffs of the plankton." Just outside of the estuary of the Mersey in Lancashire there were "not less than twenty, and not more than two hundred animals varying in size from an amphipod (one fourth inch long) to a plaice (eight to ten inches long) on every square meter of bottom" (Johnstone, 1909: 149, 176, 195-6). Finally the quantity of life in the shallow waters of the sea is not directly governed by favorable habitat, such as shallow sunlight waters in constant circulation and of equable temperature, but seems to be primarily controlled by the amount of the minimal food elements. Sea-water may be regarded as a dilute food-solution having the essential materials on which life is dependent. Of these nitrogen and the compounds of silica and phosphoric acid are present in the smallest amount. Johnstone tells us that "The density of the marine plants will therefore fluctuate according to the proportions of these indispensable food-stuffs" (234). "It is only the protophyta among the