In Life on _terra firma_, generally, the work of plants is to make ready food for animals. Many substances, which are needful for animal-structure, cannot be taken in by them until broken up and re-made by plants. And plants can only carry out this task in sunlight. Where the light of the sun fails, there plant-life fails also. But direct vegetable food is not always necessary. Arctic animals--bears, seals, walruses,--cannot get it except at second or third hand, through the bodies of other animals. Perhaps this was forgotten by some who maintained that no animals could, by any possibility, flourish in ocean’s greater depths. Numberless observations have now thrust that theory on one side. By the dredgings and trawlings of the _Challenger_, not to speak of later explorations, it has been conclusively proved that, even down to great depths, animal life is marvellously plentiful. Two or three examples may be given. A single haul, made in water more than a mile deep, brought up animals of two hundred different kinds. Another haul, in a depth of two miles, had the same result. Another, in a depth of three miles, brought up fifty different kinds. All these it was known, by tokens learnt from close study and much experience, had certainly lived near the bottom. Again, from depths of four miles many creatures, including fishes, appeared. Even in a depth of over five miles, signs of life were not lacking. Who, after this, may venture to name a depth at which Life is altogether impossible? Although plants cannot live and grow in the darkness of nether ocean, their decaying remains are incessantly raining downward from higher levels. Animals in the intermediate parts, and perhaps also near the sea-bed, can feed largely upon these falling sea-weeds. Immense quantities are doubtless snapped up _en route_; but many also reach the ocean-floor undevoured, since vegetable-remains often figure in the muds and oozes brought up thence. Deep-sea animals are very extensively eaters of mud and ooze and clay. It does not sound like appetising fare, or what our grandparents used to describe as “palatable.” But the palate of a deep-sea fish or crab differs from that of a man; and since no better vegetable-food offers itself, they are probably content. That the said creatures do really subsist in a measure upon such materials, is known for a fact. When drawn up to the surface, they are found often to have their stomachs full of the mud or ooze or clay which in that particular place carpets the ocean-bed. Even in far shallower waters, not much removed from the “hundred-fathom line,” though still beyond the region of living sea-weed, crabs dredged up are found to have indulged freely in the same frugal rations. There is of course no difficulty as to animal-food in any depth. The fight for life goes on below, as above; and the weaker succumbs to the stronger, or the less cunning to the more subtle, in the deep as within reach of sunlight. CHAPTER XIX.