whence these animalcules are called “Ciliata.” The ciliates or one-celled animals are enabled by their cilia to move with all the grace, variety, facility, and apparent intelligence of the highest animals, and also to create powerful vortex-currents by which food particles are driven into the cell-mouth. It is a most remarkable and thought-stirring fact that here we have “animalcules” which are no more than isolated units of the kind and structure which go by hundreds of thousands to build up a larger animal—just as bricks are units of the kind which to the number of many thousands build up a house. And yet each of these free-living units has a complete organisation—mouth, pharynx, renal organ, locomotive organs, and so on—similar in activity and general shape to the system of large, capacious organs built up by the agglomeration of millions of cell-units to form the body of a higher animal. It is as though a single brick were provided with door, windows, staircase, fireplace, chimneys, and wine-cellar! It is clear that there is only a resemblance and not an identity of origin between the organs of the multicellular animal and those of the single-celled animalcule. The history of the growth of an animal from the single egg-cell, and also the series of existing many-celled animals, leading from simple forms to the most complex, proves this. And in view of that fact the wonderful elaboration of these diminutive creatures—many of them so small as to be absolutely invisible to the naked eye—is all the more curious and impressive. We have, in fact, parallel organisation and elaboration of structures with special uses, in two absolutely separated grades or strata of living things—the one grade marked off by the limitation that only a single cell, a single nucleated corpuscle of protoplasm, is to be the basis and material of elaboration—the other and higher grade permitting the use of millions of single cells, of endless variety and plasticity, capable of hanging together and being grouped in layers and tissues, in such enormous masses that an elephant or a whale is the result. And we see that the same needs are met, not actually in the same way, but in the same kind of way, in the two cases—the food-orifice, the cilia, and the “pulsating vacuole” of the unicellular animalcule do the same services as those done by the structurally different mouth, legs, and kidneys of the elephant. XXII TADPOLES AND FROGS