the ear, and the fewer are the vibrations in a second, the deeper is the sound, which becomes shriller and shriller as the waves of sound become more rapid. In human ears the limits of hearing are reached when about 35,000 vibrations strike the drum of the ear in a second; but there is no reason why 50,000 or 100,000 vibrations should not be audible to other animals.

Whatever the explanation of the gift of hearing in ourselves may be, different plans seem to be adopted in the case of other animals. In many Crustacea and Insects flattened hairs each connected

there are

as

with a nerve fibre, and SO constituted to vibrate in response to particular notes. In others the ear cavity contains certain minute solid bodies, known as otoliths, which in the same way play upon the nerve fibres. Sometimes these are secreted by the walls of the cavity itself, but certain Crustacea have acquired the remarkable habit of selecting after each moult suitable particles of sand, which they pick up with their pincers and insert into their ears.

Many insects, besides the two large "compound" eyes one on each side of the head, have between them three small ones, known as the "ocelli," arranged in a triangle. The structure of these two sets of eyes is quite different. The ocelli appear to see as our eyes do. The lens throws an inverted image on the back of the eye, so that with these eyes they must see everything reversed, as we ourselves really do, though long practice enables us to correct the impression. On the other hand, the compound eyes consist of a number of facets, in some species as many as 20,000 in each eye, and the prevailing impression among entomologists now is that each facet receives the impression of one pencil of rays, that in fact the image formed in a compound eye is a sort of mosaic. In that case, vision by means of these eyes must be direct; and it is indeed. difficult to understand how an insect can obtain a correct impression when it looks at the world with five eyes, three of which see everything reversed, while the other two see things the right way up!

On the other hand, some regard each facet as an independent eye, in which case many insects realise the epigram of Plato

Thou lookest on the stars, my love,

Ah, would that I could be

Yon starry skies with thousand eyes,
That I might look on thee!

Even so, therefore, we only substitute one difficulty for another.

But this is not all. We have not only no proof that animals are confined to our five senses, but there are strong reasons for believing that this is not the case.

In the first place, many animals have organs which from their position, structure, and rich supply of nerves, are evidently organs of sensation; and yet which do not appear to be adapted to any one of our five senses.

As already mentioned, the limits of hearing are reached when about 35,000 vibrations of the air strike on the drums of our ears. Light, as was first conclusively demonstrated by our great countryman Young, is the impression produced by vibration of the ether on the retina of the eye. When 700 millions

of millions of vibrations strike the eye in a second, we see violet; and the color changes as the number diminishes, 400 millions of millions giving us the impression of red.

Between 35,000 and 400 millions of millions the interval is immense, and it is obvious that there might be any number of sensations. When we consider how greatly animals differ from us, both in habits and structure, is it not possible, nay, more, is it not likely that some of these problematical organs are the seats of senses unknown to us, and give rise to sensations of which we have no conception?

LUMINOSITY

In addition to the capacity for receiving and perceiving, some animals have the faculty of emitting light. In our country the glowworm is the most familiar case, though some other insects and worms have, at any rate under certain conditions, the same power, and it is possible that many others are really luminous, though with light which is invisible to us. In warmer climates the Fire-fly,

Lanthorn-fly, and many other insects, shine with much greater brilliance, and in these cases the glow seems to be a real love-light, like the lamp of Hero.

Many small marine animals, Medusæ, Crustacea, Worms, etc., are also brilliantly luminous at night. Deep-sea animals are endowed also in many cases with special luminous organs, to which I shall refer again.

SENSE OF DIRECTION

It has been supposed that animals possess also what has been called a Sense of Direction. Many interesting cases are on record of animals finding their way home after being taken a considerable distance. To account for this fact it has been suggested that animals possess a sense with which we are not endowed, or of which, at any rate, we possess only a trace. The homing instinct of the pigeon has also been ascribed to the same faculty. My brother Alfred, however, who has paid much attention to pigeons, informs me that they are never taken any great