Not indeed always of animal life. Sometimes vegetable life takes its place. Though the highest Vegetables stand on a lower level than the lowest Animals, they too have Life, and thereby they are entirely apart from the whole world of inanimate Nature.
Widely distinct are the Animal Kingdom and the Vegetable Kingdom; yet on the borderland between the two is found a hazy belt of uncertainty.
So gently does the one glide into the other that, though a line of demarcation does exist, it is not always easily made out. But the transition from the lowest form of that which lives to any and every form of that which has no Life is abrupt, absolute, precipitous. Here we see no quiet sliding of the one into the other, no wavering hesitancy as to whether a certain something may belong to this or that side of the dividing parapet.
The term “Vegetables,” used scientifically about Land Vegetation, includes all manner of growths, from the lichen to the forest tree. But by that word, used in connection with the Ocean, is meant, not cabbages and cauliflowers, not garden-plants or forest-trees, but an enormous variety of Sea-Weeds.
And among ocean-vegetables of humblest form, flourishing in salt water, is that of Diatoms—Flint-makers.
It need not be imagined that the manufacture of Flint is a monopoly of the Diatom tribe. That would be a mistake.
We have had to do in the last chapter with small carbonate-of-lime building jelly-specks. We shall have to do in the next chapter with carbonate-of-lime building jelly-polyps. But there are also little vegetable-growths in the sea which form carbonate-of-lime, and some of these add their quota to the work of rearing Coral-reefs.
We have to do in the present chapter with small flint-building vegetable-growths. But there are also little animal jelly-specks in the sea which form flint.
So neither Carbonate-of-lime nor Flint can be spoken of as belonging only to Animal or only to Vegetable Life. They must both be described in general terms as the outcome of that which Lives.
Among other Flint-builders in the ocean are the Radiolarians; living jelly-specks, and makers of the most exquisite shells, so tiny in size that millions of them may be packed into one cubic inch. These belong to the Animal Kingdom. They rank with the Foraminifera, differing from them chiefly in the fact that their shells or skeletons are formed of silica, not carbonate-of-lime.