non-living, so the bones and skin which protoplasm is supposed to construct are held non-living and the protoplasm alone living.  Protoplasm, it is said, goes about masked behind the clothes or habits which it has fashioned.  It has habited itself as animals and plants, and we have mistaken the garment for the wearer—­as our dogs and cats doubtless think with Giordano Bruno that our boots live when we are wearing them, and that we keep spare paws in our bedrooms which lie by the wall and go to sleep when we have not got them on.

If, in answer to the assertion that the osseous parts of bone are non-living, it is said that they must be living, for they heal if broken, which no dead matter can do, it is answered that the broken pieces of bone do not grow together; they are mended by the protoplasm which permeates the Haversian canals; the bones themselves are no more living merely because they are tenanted by something which really does live, than a house lives because men and women inhabit it; and if a bone is repaired, it no more repairs itself than a house can be said to have repaired itself because its owner has sent for the bricklayer and seen that what was wanted was done.

We do not know, it is said, by what means the structureless viscid substance which we call protoplasm can build for itself a solid bone; we do not understand how an amoeba makes its test; no one understands how anything is done unless he can do it himself; and even then he probably does not know how he has done it.  Set a man who has never painted, to watch Rembrandt paint the Burgomaster Six, and he will no more understand how Rembrandt can have done it, than we can understand how the amoeba makes its test, or the protoplasm cements two broken ends of a piece of bone.  Ces choses se font mais ne s’expliquent pas.  So some denizen of another planet looking at our earth through a telescope which showed him much, but still not quite enough, and seeing the St. Gothard tunnel plumb on end so that he could not see the holes of entry and exit, would think the trains there a kind of caterpillar which went through the mountain by a pure effort of the will—­that enabled them in some mysterious way to disregard material obstacles and dispense with material means.  We know, of course, that it is not so, and that exemption from the toil attendant on material obstacles has been compounded for, in the ordinary way, by the single payment of a tunnel; and so with the cementing of a bone, our biologists say that the protoplasm, which is alone living, cements it much as a man might mend a piece of broken china, but that it works by methods and processes which elude us, even as the holes of the St. Gothard tunnel may be supposed to elude a denizen of another world.