_Life on the Continental Shelf._--No attempt need be made here to give an accurate description of the plants and animals which find a congenial home on the continental shelf. One needs, however, to have in mind a general idea of the wonderful abundance and variety of organic forms in the shallow waters adjacent to the continent to fully appreciate the changes in the relief of the ocean-floor, in which they play a leading part, and also the vast economic importance of these marine harvest-fields and pastures, as they may be termed.
The continental shelf fringing North America reaches from within 500 miles of the equator to probably a less distance of the north pole. Its length following its broader curves is in the neighbourhood of 25,000 miles, and its average width is not far from 50 miles; its area is therefore something like 1,000,000 square miles. On account of the vast extent of this submarine plain and its great range in latitude, the conditions influencing the lives of the plants and animals inhabiting its surface or living in the waters covering it vary from place to place between wide extremes. The waters resting on it have a mean annual surface temperature of from 70° to 80° F. at the south, and about 32° F. at the north. In places great rivers and the turbid waters from glaciers bring in sediments and form muddy deposits; at other localities the currents, as in the path of the Gulf Stream off the Carolina coast, sweep the bottom clear of all light _débris_; and again bare rocks of limited extent are exposed. The depth of the water resting on the shelf varies through all gradations down to 100 fathoms. At the south the hours of light and darkness are approximately equal each day, but at the north there are six months of sunlight and six months of darkness each year. Still other variations, as of strength of currents, salinity, etc., exert an influence in this realm and lead to great diversity in its living organisms. Throughout its entire extent, however, the continental shelf abounds in both plant and animal life.
The plant life of the sea, as has been shown in recent years by the use of the dredge and net, is most abundant at the surface and is practically absent at the bottom where the water is over 100 fathoms deep. The animals of the sea, like those of the land, are dependent primarily on plants for their food. By far the most abundant supply of food plants in the sea is furnished by minute algæ, which float free in its water. Below a depth of about 100 fathoms algæ are absent because of lack of light, and all the deep-sea animals are believed to be carnivorous. For these reasons the comparatively shallow waters adjacent to the land and mainly covering the continental shelf are the most favourably circumstanced of any portion of the sea for the support of a teeming fauna.