Читать книгу Bombay Ducks - Douglas Dewar - Страница 9

Table of Contents

The late Richard Jefferies once defined man as “an animal with arms.” The definition, so far as it goes, is a good one, for it is to his arms, quite as much as to his superior brain, that man owes his present supreme position at the head of animal creation.

So much has been written regarding the large brain of man that the other factor which has contributed to his triumph is in danger of being utterly neglected. The arms and brain of man are the two physical necessities to him as a species; take away either, and he becomes something else. To endeavour to decide which of the two organs is the more useful would be as futile as to attempt to prove that the right wheel is more essential to a dog-cart than the left.

Consider what a helpless creature man would be were his arms replaced by a second pair of legs. We human beings would still be dwellers in caves, living in terror of the lion, the tiger, the wolf, and the wild boar. On the other hand, arms, without a suitable brain, will not make a man; for monkeys have arms.

Since the rest of the animals do not possess these organs, they must be very helpless creatures as compared with man. This they indeed are, but not so helpless as might at first be supposed, because they have other compensating organs.

The elephant possesses a trunk which is nearly as useful as an arm. The sensitive upper lip of the horse, the tapir, and other creatures, is a rudimentary prehensile organ—an attempt at a hand. The beak of the parrot, the crow, and the woodpecker, and the claws of most birds perform many of the functions of the human hand. The fore-limbs of some mammals, as, for instance, the bear and the squirrel, are utilized in a similar way.

In addition to these auxiliaries nearly every vertebrate animal boasts a tail. To the naturalist this is perhaps the most interesting of all organs. It is one of the few luxuries which parsimonious Dame Nature allows her children. Always a useful organ, the tail is in hardly a single instance absolutely essential to the existence of its possessor. I doubt if any animal exists that could not manage to jog along through life without its caudal appendage.

The organ seems, so to speak, to have arisen by accident. Without desiring to dogmatise, I think it may be laid down that the early ancestors of the vast majority of existing back-boned animals were amphioxus-like creatures devoid of limbs. When these appendages first budded forth it chanced that the hind pair did not arise at the extreme end of the animal; they took origin some little way forward. And, as the vital organs did not extend to the whole length of the body, there remained a posterior portion of comparative unimportance to its possessor—a quantity of plastic substance capable of being moulded into almost any shape and utilized in all manner of ways.

The fish and the whale needed a propelling organ to enable them the more rapidly to force their way through the water; the tail was pressed into service. The squirrel and the fox felt the want of a warm counterpane to protect them from the chilly blasts of the cold east winds, so Nature took the plastic tail, lengthened it, covered it with thick, soft, fluffy fur, and thus presented the animals with warm quilts.

In other cases Nature has made the tail into a prehensile organ, so that its possessors have become very expert tree-climbers, and are also able to utilize the caudal appendage in carrying their young.

Some creatures inhabit damp marshes and hot countries where flies abound, ready to sting them and worry them to death. A fly-whisk is almost a necessity to such animals, so Nature has made one for them out of their tail.

The skunk hit upon a strange mode of keeping off his enemies. He devised the plan of secreting a fluid emitting the most disgusting odour, so powerful that no animal will willingly venture near him. He needed an advertisement of this fact, lest some animal should attack him in mistake for an inoffensive creature, so his tail was converted into an advertisement board. He trots along slowly with his caudal appendage aloft, and every animal recognizes it, so he is allowed to pass through life unmolested.

The tail is a conspicuous feature in the anatomy of birds. Most of the fowls of the air are able to boast of a caudal appendage of sorts. Some possess resplendent tails—the products, we are told, of sexual selection, the admiration of the ladies for that which is beautiful. In very many cases the tail acts as a rudder or steering apparatus to its possessor during flight.

This is well seen in the king-crow, the swift, the swallow, and, indeed, in most fly-catching species. The tail feathers of the woodpecker are very stiff and are of great use in helping the bird to maintain its position on the trunks of trees. In nearly every case the tail is of use during the flight of its possessor. Nine birds out of ten spread out their caudal feathers when they take to their wings. The feathers of a bird’s tail are arranged so that the tail is almost impervious to air. They are, moreover, provided with powerful muscles, so that when the bird flies they can be spread out in the shape of a fan with a curved surface, the concavity being underneath. This is especially well seen in the flight of a dove or a kite. Nevertheless, the tail is not indispensable to a flying bird.

I once cut off, quite close to their bases, the tail feathers of a pigeon; the bird flew quite easily after the operation. The motion of the wings was perhaps rather more rapid, and the flight generally more laboured; nor did the bird steer itself so well as usual. Therefore, the tail, although both an ornamental and useful organ, is by no means indispensable to a bird. As has before been remarked, the caudal appendage is one of the few luxuries which Nature allows her children.

In the case of some animals, the use of the tail is not so obvious. Take the lizard as an example. His tail would appear at first sight to be of little or no service to him, since he parts with it so readily. As a matter of fact, the little reptile has many enemies; of these, the Indian crow is the chief. Now, when a crow attacks a lizard, it naturally tries to seize him somewhere near the middle. While the bird is striking at him, the reptile starts to run away; the result is that the crow either misses him or seizes him by the tail. If the latter happens, the tail is swiftly detached, and the lizard makes good his escape.

A few animals possess tails which apparently serve no useful purpose. These are exceedingly interesting creatures, for, if their tails really are useless, they are anomalies that threaten to upset all the theories of biological science. I do not know the use of the tail of the rat, or the mouse. Yet we may be tolerably certain that in each case the organ has some use or it would not exist. I employ the word “use” in a very wide sense. I hold an organ to be useful to an animal if it help its possessors to obtain a mate.

Galton maintains that the action of Natural (or Sexual) Selection is necessary to keep any organ up to the mark; that if the action of Natural Selection is removed from any organ, that organ at once begins to deteriorate. In other words, from the moment an organ becomes useless to its possessor, that organ begins to degenerate, and eventually disappears. Proofs of this are seen throughout the realm of nature.

Many animals which spend their lives in utter darkness, whether in the depths of the ocean, or in caves, have lost not only their sight, but even their eyes. Man’s tail became useless to him, so has disappeared. The whale’s legs were no longer needed when it took to an aquatic life; they were, therefore, transformed into fins.

Thus it is probable that, if the tails of the mouse and the rat served no useful purpose, these animals would long ago have been reduced to the state of the guinea-pig. What, then, is the use of the tail in each case? This is, indeed, a problem. These creatures, being nocturnal in their habits, do not afford the naturalist many opportunities of watching them. Nevertheless, they move in such a rapid, silent, mysterious way that it is more than possible that the long supple tail assists them during locomotion.