What then determines the difference between different animals? What makes one little speck of protoplasm grow into Newton's dog Diamond, and another, exactly the same, into Newton himself? It is a mysterious something which has entered into this protoplasm. No eye can see it. No science can define it. There is a different something for Newton's dog and a different something for Newton; so that though both use the same matter they build it up in these widely different ways. Protoplasm being the clay, this something is the Potter. And as there is only one clay and yet all these curious forms are developed out of it, it follows necessarily that the difference lies in the potters. There must in short be as many potters as there are forms. There is the potter who segments the worm, and the potter who builds up the form of the dog, and the potter who moulds the man. To understand unmistakably that it is really the potter who does the work, let us follow for a moment a description of the process by a trained eye-witness. The observer is Mr. Huxley. Through the tube of his microscope he is watching the development, out of a speck of protoplasm, of one of the commonest animals: "Strange possibilities," he says, "lie dormant in that semi-fluid globule. Let a moderate supply of warmth reach its watery cradle and the plastic matter undergoes changes so rapid and yet so steady and purposelike in their succession that one can only compare them to those operated by a skilled modeler upon a formless lump of clay. As with an invisible trowel the mass is divided and subdivided into smaller and smaller portions, until it is reduced to an aggregation of granules not too large to build withal the finest fabrics of the nascent organism. And, then, it is as if a delicate finger traced out the line to be occupied by the spinal column, and moulded the contour of the body; pinching up the head at one end, the tail at the other, and fashioning flank and limb into due proportions in so artistic a way, that, after watching the process hour by hour, one is almost involuntarily possessed by the notion, that some more subtle aid to vision than an achromatic would show the hidden artist, with his plan before him, striving with skillful manipulation to perfect his work."[87]
Besides the fact, so luminously brought out here, that the artist is distinct from the "semi-fluid globule" of protoplasm in which he works, there is this other essential point to notice, that in all his "skillful manipulation" the artist is not working at random, but according to law. He has "his plan before him." In the zoological laboratory of Nature it is not as in a workshop where a skilled artisan can turn his hand to anything--where the same potter one day moulds a dog, the next a bird, and the next a man. In Nature one potter is set apart to make each. It is a more complete system of division of labor. One artist makes all the dogs, another makes all the birds, a third makes all the men. Moreover, each artist confines himself exclusively to working out his own plan. He appears to have his own plan somehow stamped upon himself, and his work is rigidly to reproduce himself.
The Scientific Law by which this takes place is the Law of Conformity to Type. It is contained, to a large extent, in the ordinary Law of Inheritance; or it may be considered as simply another way of stating what Darwin calls the Laws of Unity of Type. Darwin defines it thus: "By Unity of Type is meant that fundamental agreement in structure which we see in organic beings of the same class, and which is quite independent of their habits of life."[88] According to this law every living thing that comes into the world is compelled to stamp upon its offspring the image of itself. The dog, according to its type, produces a dog; the bird a bird.
The artist who operates upon matter in this subtle way and carries out this law is Life. There are a great many different kinds of Life. If one might give the broader meaning to the words of the apostle: "All life is not the same life. There is one kind of life of men, another life of beasts, another of fishes, and another of birds." There is the Life, or the Artist, or the Potter who segments the worm, the potter who forms the dog, the potter who moulds the man.[89]
What goes on then in the animal kingdom is this--the Bird-Life seizes upon the bird-germ and builds it up into a bird, the image of itself. The Reptile Life seizes upon another germinal speck, assimilates surrounding matter, and fashions it into a reptile. The Reptile-Life thus simply makes an incarnation of itself. The visible bird is simply an incarnation of the invisible Bird-Life.