Life, from its very nature, cannot mean stagnation, or standing still. It must always be assimilating. It must always be expanding. It must always be doing. When these things cease, death has begun. So much alike are the lower forms of life in the two classes, that many an animal has been for a time mistakenly called a vegetable, and many a vegetable has been for a while supposed to be an animal. The foraminifera were once ranked as vegetables, and the diatoms were ranked as animals. In deep-sea regions, dark and cold, no vegetable life can possibly exist, not even the almost ubiquitous diatoms. But their remains are present in vast multitudes. As the tiny plant-life dies out, the little hard vegetable-cases sink to the ocean-bed, there to mingle with gathering muds and oozes--there, too, in the course of ages, to be transformed into Flint. Diatoms flourish on land as well as in the ocean; in cold climates as well as in warm. On the whole they prefer the colder regions, therein differing from the warmth-loving foraminifera. Sometimes immense floating banks of these microscopic plants are found, reaching through many miles. A net, lowered into such a bank and then drawn up, is filled with a “brown-coloured slimy and felt-like mass,” made up chiefly of uncountable myriads of diatoms. Such masses are held together by a kind of sticky jelly, and if the hand is passed through it a very slight roughness may be perceptible, caused by the flinty diatom shells or cases. Mere specks indeed they are, individually too minute to be seen by a human eye, unaided. Yet how marvellous in their make! A Diatom plant or vegetable or sea-weed, whichever we choose to call it, is like a Foraminifer of the simplest possible structure. It consists of a single cell, with an outside flinty coating or inclosure, answering to the wood of a tree or the skeleton of an animal. This flint casing, though infinitely small and delicate, is yet firm enough to prevent the passage of water through it; and also it is practically indestructible by ordinary forces. Diatom-cases, dropping to the ocean-bed, may lie there for ages. [Illustration: DIATOM CASES (ARACHNOIDISCUS JAPONICUS) _Reproduced by permission of Messrs. R. and J. Beck, Ltd., and Messrs. J. and A. Churchill, publishers of Carpenter’s “The Microscope”_ _Face page 154_ ] The living cell is inclosed in the said “case,” which is a kind of box, consisting of two halves or “frustules,” neatly joined together by a ring or girdle. Diatoms increase in numbers, like many of the Foraminifera, by simply dividing into two. When the living cell thus parts, each half takes as its share one side of the box, and each then makes another side, or “frustule,” to complete itself. For this operation, the tiny ring or hoop doubles into a pair of hoops, one of which clings to each half of the case. Perhaps in all the world no greater marvels are to be seen than these extraordinary minute vegetable-cases.