Part 4 (1/2)
Life is indeed among animals a struggle for existence, and in addition to the more usual weapons--teeth and claws--we find in some animals special and peculiar means of offence and defence.
If we had not been so familiarised with the fact, the possession of poison might well seem a wonderful gift. That a fluid, harmless in one animal itself, should yet prove so deadly when transferred to others, is certainly very remarkable; and though the venom of the Cobra or the Rattlesnake appeal perhaps more effectively to our imagination, we have conclusive evidence of concentrated poison even in the bite of a midge, which may remain for days perceptible. The sting of a Bee or Wasp, though somewhat similar in its effect, is a totally different organ, being a modified ovipositor. Some species of Ants do not sting in the ordinary sense, but eject their acrid poison to a distance of several inches.
Another very remarkable weapon is the electric battery of certain Eels, of the Electric Cat Fish, and the Torpedoes, one of which is said to be able to discharge an amount of electricity sufficient to kill a Man.
Some of the Medusae and other Zoophytes are armed by millions of minute organs known as ”thread cells.” Each consists of a cell, within which a firm, elastic thread is tightly coiled. The moment the Medusa touches its prey the cells burst and the threads spring out. Entering the flesh as they do by myriads, they prove very effective weapons.
The ink of the Sepia has pa.s.sed into a proverb. The animal possesses a store of dark fluid, which, if attacked, it at once ejects, and thus escapes under cover of the cloud thus created.
The so-called Bombardier Beetles, when attacked, discharge at the enemy, from the hinder part of their body, an acrid fluid which, as soon as it comes in contact with air, explodes with a sound resembling a miniature gun. Westwood mentions, on the authority of Burch.e.l.l, that on one occasion, ”whilst resting for the night on the banks of one of the large South American rivers, he went out with a lantern to make an astronomical observation, accompanied by one of his black servant boys; and as they were proceeding, their attention was directed to numerous beetles running about upon the sh.o.r.e, which, when captured, proved to be specimens of a large species of Brachinus. On being seized they immediately began to play off their artillery, burning and staining the flesh to such a degree that only a few specimens could be captured with the naked hand, and leaving a mark which remained a considerable time.
Upon observing the whitish vapour with which the explosions were accompanied, the negro exclaimed in his broken English, with evident surprise, 'Ah, ma.s.sa, they make smoke!'”
Many other remarkable ill.u.s.trations might be quoted; as for instance the web of the Spider, the pit of the Ant Lion, the mephitic odour of the Skunk.
SENSES
We generally attribute to animals five senses more or less resembling our own. But even as regards our own senses we really know or understand very little. Take the question of colour. The rainbow is commonly said to consist of seven colours--red, orange, yellow, green, blue, indigo, and violet.
But it is now known that all our colour sensations are mixtures of three simple colours, red, green, and violet. We are, however, absolutely ignorant how we perceive these colours. Thomas Young suggested that we have three different systems of nerve fibres, and Helmholtz regards this as ”a not improbable supposition”; but so far as microscopical examination is concerned, there is no evidence whatever for it.
Or take again the sense of Hearing. The vibrations of the air no doubt play upon the drum of the ear, and the waves thus produced are conducted through a complex chain of small bones to the fenestra ovalis and so to the inner ear or labyrinth. But beyond this all is uncertainty. The labyrinth consists mainly of two parts (1) the cochlea, and (2) the semicircular ca.n.a.ls, which are three in number, standing at right angles to one another. It has been supposed that they enable us to maintain the equilibrium of the body, but no satisfactory explanation of their function has yet been given. In the cochlea, Corti discovered a remarkable organ consisting of some four thousand complex arches, which increase regularly in length and diminish in height. They are connected at one end with the fibres of the auditory nerve, and Helmholtz has suggested that the waves of sound play on them, like the fingers of a performer on the keys of a piano, each separate arch corresponding to a different sound. We thus obtain a glimpse, though but a glimpse, of the manner in which perhaps we hear; but when we pa.s.s on to the senses of smell and taste, all we know is that the extreme nerve fibres terminate in certain cells which differ in form from those of the general surface; but in what manner the innumerable differences of taste or smell are communicated to the brain, we are absolutely ignorant.
If then we know so little about ourselves, no wonder that with reference to other animals our ignorance is extreme.
We are too apt to suppose that the senses of animals must closely resemble, and be confined to ours.
No one can doubt that the sensations of other animals differ in many ways from ours. Their organs are sometimes constructed on different principles, and situated in very unexpected places. There are animals which have eyes on their backs, ears in their legs, and sing through their sides.
We all know that the senses of animals are in many cases much more acute than ours, as for instance the power of scent in the dog, of sight in the eagle. Moreover, our eye is much more sensitive to some colours than to others; least so to crimson, then successively to red, orange, yellow, blue, and green; the sensitiveness for green being as much as 750 times as great as for red. This alone may make objects appear of very different colours to different animals.
Nor is the difference one of degree merely. The rainbow, as we see it, consists of seven colours--red, orange, yellow, green, blue, indigo, and violet. But though the red and violet are the limits of the visible spectrum, they are not the limits of the spectrum itself, there are rays, though invisible to us, beyond the red at the one end, and beyond the violet at the other: the existence of the ultra red can be demonstrated by the thermometer; while the ultra violet are capable of taking a photograph. But though the red and violet are respectively the limits of our vision, I have shown[16] by experiments which have been repeated and confirmed by other naturalists, that some of the lower animals are capable of perceiving the ultra-violet rays, which to us are invisible. It is an interesting question whether these rays may not produce on them the impression of a new colour, or colours, differing from any of those known to us.
So again with hearing, not only may animals in some cases hear better than we do, but sounds which are beyond the reach of our ears, may be audible to theirs. Even among ourselves the power of hearing shrill sounds is greater in some persons than in others. Sound, as we know, is produced by vibration of the air striking on the drum of 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.
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 there are flattened hairs each connected with a nerve fibre, and so const.i.tuted as 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 subst.i.tute 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.