OVER-EQUIPPED ANIMALS.
ALECTURE recently given at the Royal Institution by Dr. Fraser on the medical treatment of the bite of venomous serpents fully confirms the popular estimate of the deadly nature of the weapon carried by these serpents. The minimum lethal dose varies in proportion to the size, and also to the individuality of the creature bitten, for though more poison is needed to kill a man than to kill a cat, the cat needs more in proportion to its weight. But non- venomous snakes are the only creatures which do not succumb almost instantly to the poison, except the venomous serpents themselves. The cobra is proof against cobra poison, and the puff-adder against the bite of another puff- adder, apparently because all the food which they eat is the flesh of animals which the snakes themselves have killed with their own poison. The best safeguard for the human being is to obtain immunity in the same way, by swallowing the poison. The selective power of the stomach retains for use in the system the protecting element and rejects the rest, and it has been found that a kitten suckled by a mother cat which had been "immunised" by swallowing cobra poison was also protected. Hypodermic injection is mainly in- effectual, because of the frightful power of the poison and the microscopic doses which must be used, the amount con- tained in one or two test-tubes shown by the lecturer being probably more than enough to have killed the entire audience assembled.
The testimony to the deadly nature of the serpent's weapon is so complete that the question naturally occurs why the non-venomous serpents have survived at all in the struggle for the survival of the fittest. The problem is attractive mainly because the conditions of snake-structure seem almost intended as an illustration of what we may call dogmatic evolution. Certain snakes, of medium size, carry the most deadly weapon, whether of offence or defence, owned by any animal. In addition, they have the perfect vertebrate structure which Sir Richard Owen so much admired, and can therefore swim, climb, and make their way in places where no animal of similar size can go. They are the best- equipped animals of their kind. Side by side with these are found snakes of similar size and equal powers, except that they lack the invincible lethal weapon. They have developed all the special attributes of their kind except that one which renders them an object of terror to all other creatures. They only carry blank-cartridge in the battle of life. Evolution seems arrested for no reason ; they not only fail to grow the poison-tooth, but do not develop the power of crushing their prey as the constrictors do. It would be far easier for a common grass-snake to kill a frog by poison before eating it, as the cobra does, than to have to swallow it alive. It would save it trouble to crush it, as a young boa constrictor of the same size would do; but it does neither, because it neither has developed, nor seems in the least likely to develop, the necessary equipment for so doing.
By the theory of evolution the grass-snake ought therefore to go back in the world, while the other common English snake, the poisonous viper, armed with a weapon equally power- ful as a protection and a means of killing prey, in. creased in size and numbers, and took its place. Yet this is exactly what has not happened. Grass-snakes are larger and more numerous than adders, and there is no evidence that the absence of the poison-fangs in any way endangers the survival of the species. Instances of this disparity of equip- ment, seen in such a marked form in the case of the venomous and non-venomous snakes, are commonly found among other animals, and though by the analogy of certain examples of evolution the less well-provided ought to dis- appear, or to develop better physical powers and appli- ances, they manage to maintain their place without
modifications of their structure. There is hardly any large group of birds, beasts, fishes, or insects in which instances do not occur of the successful survival of creatures which have never acquired the specialised powers gained by others; and when creatures of wholly different orders, but living upon identical food, are compared, such as the birds, fish, monkeys, and reptiles which feed upon insects, the disparity of equipment is even more striking. It is not difficult to ac- count for the survival of the non-venomous snakes, except on the supposition that the poison is a necessity for obtaining food for self-defence, which the continued existence of the non- poisonous snake shows that it is not. But it is difficult to account for the absence of any visible failure in life of insect- feeding creatures competing for existence on the same food, but with such unequal physical means for obtaining it, as are pos- sessed by the swallow, the marmoset, the shrew, the chameleon, and the frog. In this list of five creatures living upon similar food we find that the physical appliances for obtaining it range from the perfect development of speed in flight in the swallow, the addition of hands for capture in the marmoset, bodily quick- ness and activity in the shrews, the power of assuming colour exactly similar to that of environment in the chameleon, supplemented by a special apparatus for shooting out the tongue and capturing insects when motionless, to the absence of any special equipment at all, except that of a rather long tongue, in the insect-eating frog. Yet the frog, destitute of all these specialised appliances, lives just as well as the swallow, the shrew, the marmoset, or the chameleon, and at least one species, the barking-frog, finds that to get on in the world it has only to be still and wait till creatures walk into its mouth. It has acquired a skin-colour suited to its environment, and a large mouth. But it is not, and could never be, considered well provided for getting its living.
Our estimate of the physical means and appliances necessary for the survival and well-being of animals is probably set too high. We judge the needs of all from the perfect develop. ment and acquired powers of many, perhaps of most, which evolution has provided with appliances in excess of their real wants. Most animals are over-equipped. Evolution has run riot, and provided them with means and metal far in excess of their needs, just as it has provided them with an exuberance of ornament which delights us, but must already have passed beyond animal comprehension. Probably the cats, great and small, would at once be named as the best equipped of any class of predatory creature, and the predatory creatures are by necessity superior in most forms of physique to those on which they prey. They are all " built " on one plan, with a special armament of teeth, sheaths to keep their claws sharp, muscles for springing suddenly to great distances, padded feet to deaden the sound of their movements, and colour adapted for concealment. These acquired appliances are in excess of their wants. The polar bear, which cannot spring, and has no sheaths for its claws, and has little obvious pro- vision to aid it in swimming, catches and kills animals larger than those killed by the tiger, and can kill them in the water. The wolverine or glutton, which is heavy, slow, and has the feet of a ferret, gets its living as a carnivorous animal no less well than the tiger-cats, which have a far more specialised equipment for their work. There is actual evidence from the fossil bones of tigers that this excess of equipment once went further, and has been discarded as superfluous. An extinct species developed canine teeth of such enor- mous size that it has been named the " sabre-toothed tiger." The teeth were too long for its jaws, and the modern species use shorter weapons, just as modern ironclada are provided with lighter guns. The curling tusks of some of the mammoths and the palmated horns of the Irish elk were also in excess of requirements, and are modified for use in the existing elephants and reindeer. Many monkeys possess a thumb, but some do not, and there is no evidence that they are therefore at a disadvantage for their non-progressive life. In the case of the wingless birds—the auks, penguins, dodos, and apteryxes—there was no surplus of equipment. It was only sufficient for their maintenance before man, and other predatory animals, attacked them. But such instances of under-equipment are rare, and the preponderance lies wholly in the opposite direction. While spiders not only have poison-fangs, but also spin a web, others exist equally well which make no web at all. Perhaps a mere striking instance
of surplus equipment is that of the fruit-eating bats. The bat -which catches flying insects needs wings. But there is a large class of bats found in all tropical countries which live upon fruit. It would be quite enough for their needs to be able to climb the trees in order to eat the guavas, plantains, and rose-apples. But their wings are larger and stronger than those of the insect-eating bats. The heads of these bats are so modified that they resemble those of a dog or a fox, while their wings, which might have been expected to become atrophied, and replaced by climbing limbs, have acquired additional power. An " all-round" equipment is rare in Nature. But for the limited purposes of securing a suffi- ciency of the food necessary for the existence of a species, the provision is nearly always ample, and more often enor- mously in excess of its probable requirements.
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