birds, and they will fight with each other to the death.
The two terns are the White-rumped tern (Sterna hirundinacea), which breeds plentifully on Antarctic islands, and the Arctic tern (S. macrura). Mr. Eagle Clarke is of opinion that the Arctic tern does not breed in the Antarctic Regions, but that it is a summer visitor during the Arctic winter. Mr. Clarke says, "The finding of this tern in the seas off the South Polar continent must be regarded as one of the most important ornithological discoveries made by the Expedition (Scotia), for, as has already been stated, no terns appear to have been previously captured within seas girdled by the Antarctic Circle."
But besides whales, seals, and birds, Polar seas teem with lowlier forms of animal life from fishes down to simple unicellular animals, and it is all this vast host of fishes and invertebrates that accounts for the large number of mammals and birds in Polar Regions—north and south. These lowlier and mostly smaller forms of animal life depend, as already indicated, upon the meadows and pastures of the oceans which are made up of immense quantities of unicellular algæ. Fishes and invertebrates occur everywhere in Polar seas, from the surface down to depths of about 2,000 fathoms in the Arctic Regions, and to depths exceeding 3,000 fathoms in the Antarctic Regions.
