As it is only within the last thirty years that machinery suitable for deep-sea dredging has been invented, so it is only lately that we have been able to learn much as to the population of the ocean beneath the surface layer and marginal shallows. Now by means of beam-trawls, dredges, tangle-bars, etc., worked by steam-machinery on shipboard, naturalists may scrape up the bottom-ooze and obtain living objects or their bony relics at the depth of even 3000 to 4000 fathoms or more than four miles, for living beings are found in these profound abysses. Many scientific expeditions, such as those of the English exploring steamer _Challenger_, about 1874, have carried out these dredging investigations, and the United States Fish Commission possesses the large, specially built, sea-going _Albatross_, provided with all the necessary apparatus for deep-sea exploration. By means of these and other vessels an enormous amount of study—all useful in ascertaining the habits and methods of reproduction of food-fishes—has been carried on by American marine naturalists. It appears that as you go further and further from shore, and into deeper and deeper water, the fewer animals and plants are obtained, and that very few species indeed which live along shore are to be found also at a depth greater than about 100 fathoms. [Illustration: LANDING THE BEAM-TRAWL ON DECK.] Almost all animals, moreover, have a limited distribution in the sea, as is the case among those on land, though we cannot always, or perhaps often, say why the limits we find should exist; one sort of crab, or mollusk, or polyp, appearing _here_ and another different one exclusively _there_, when the conditions seem to us very similar, and no barrier is perceptible. It is not easy to explain why a certain sort of cowry, for example, should be found only along a particular strip of coast, when nothing that we can see prevents its extending its range much further. It is believed that the _temperature_ of the water is the chief fact which sets these invisible boundaries to the wanderings of animals living near the surface, only a few of which are very wide-spread in their distribution. The direction and character of the ocean currents have much to do with the geographic distribution of oceanic life, as has been mentioned in Chapter II (page 25). Now in deep-sea life the case is different. Here temperature cannot be of so much account, since only a short distance down, the water becomes almost as cold as ice, and preserves this uniform chill all around the globe. The life found at a great depth, too, is very wide-spread, instead of restricted in its range, often occurring in two or more ocean basins; but here the restriction is an up-and-down one, rather than horizontal, and the secret is found in the word _pressure_. Few animals are able to live both in the shallows and under the enormous weight of sea water three or four miles deep. [Illustration: A TYPICAL JELLYFISH.