have noticed how the common stone-crops will grow and root from any little scrap or fragment or bit that falls by accident upon damp soil. If we go down to the very bottom of the matter, it is clear that the plant tends to reproduce itself, whole and complete, from every part of itself—tends to increase in its own shape, and repeat itself anew in fresh leaves and branches. Why is this? Well, such a tendency results necessarily from the fundamental principle of cell-growth. Every living vegetable cell containing chlorophyl is always producing within itself fresh vital matter of its own kind; and this vital matter, at last outgrowing the capacity of the mother-cell, pushes itself out through the cell-wall, and grows into a new cell like the one it left. And it does so in the very last resort in virtue of that curious chemical property of the stuff we call chlorophyl, whereby such chlorophyl, under the influence of sunlight, separates the carbon and oxygen of carbonic acid, and builds them up once more into living matter of the particular sort composing the plant in which it exists.
Given a chemical body which can so increase the sum-total of living matter, and there must needs result the phenomenon of growth. Living matter is always being made anew from the non-living. But observe that in each plant the material thus assimilated from the air (or rather the carbonic acid floating in it), and more remotely from the earth and water, is built up into the forms of the particular plant itself—becomes distinctively, not mere living matter in the abstract, but strawberry matter, or stone-crop matter, or cactus matter, or whatever else the individual plant may happen to be. In this we get the real secret of like reproducing like. It results as a corollary from the principle of assimilation. Most people see a mystery in the particular fact that offspring resemble parents, but they see no mystery in the general fact that the parent reproduces or renews the parts of itself from alien material. In reality, the final explanation lies on this deeper and more essential level. It is just as strange that a rose should put out fresh leaves and shoots as that its seed should grow up into a fresh rose-bush.
The true explanation seems to be, as Mr. Herbert Spencer long ago suggested, that each organism has an inherent physical tendency (of the nature of polarity) to complete its own organic form, in somewhat the same way as a broken crystal, placed in a solution of its own material, has a tendency to replace its lost portions. The organic type, in other words, resembles the crystalline in this—that the material of which it is composed, when left to its own internal forces, tends, under the free play of those forces alone, to arrange itself in a certain definite specific shape.
In time, however, every organism or colony of organisms seems to lose this primitive plastic power of producing fresh
