seen in its earliest beginnings in the Discophora and the worms.
The line of argument here applied to the eye may be followed in every instance of so-called design. The exquisite mechanism of the ear may be similarly traced, from the mere sac with otoliths of the Medusæ up to the elaborate external, middle, and internal ears of man. Man's ear is a very complex thing. Its three chambers; the curious characteristics of the innermost of these, with its three "semi-circular canals", its coiled extension, like a snail-shell, called the cochlea, its elaborate nervous mechanism; the membrane between the middle and outer chambers, which vibrates with every pulsation of the air; we can trace all these separate parts as they are added one to one to the auditory apparatus of the evolving race. If we examine the edge of the "umbrella" of the free-swimming Medusa, we shall find some little capsules containing one or more tiny crystals, the homologues of the inner ear; the lower forms of Vermes have similar ears, and in some there are delicate hairs within the capsule which quiver constantly; the higher worms have these capsules paired and they lie close to a mass of nervous matter. Lobsters and their relations have similar ears, the capsule being sometimes closed and sometimes open. In many insects a delicate membrane is added to the auditory apparatus, and stretches between the vesicle and the outer air, homologue of our membrane. The lower fishes have added one semi-circular canal, the next higher two, and the next higher three: a little expansion is also seen at one part of the vesicle. In the frogs and toads this extension is increased, and in the reptiles and birds it is still larger, and is curled a little at the further end. In the lowest mammals it is still only bent, but in the higher it rolls round on itself and forms the cochlea. The reptiles and birds have the space developed between the vesicle and the membrane, and so acquire a middle ear; the crocodile and the owl show a trace of the external ear, but it is not highly developed till we reach the mammals, and even the lowest mammals, and the aquatic ones, have little of it developed. Thus step by step is the ear built up, until we see it complete as a slow growth, not as an intelligent design.
And if it be asked, how are these changes caused, the answer comes readily: "By variation and by the survival of
