The Meaning of Evolution Part 3

After the battle for food comes the struggle for shelter. For most animals there is no such thing as shelter. They are exposed to the inclemencies of the weather and to the depredations of their enemies without the means of retiring into any situation which might protect them. In the higher animals, especially when they are warmer blooded and their bodies must be kept at a higher temperature, some form of covering has come to be almost universal.

Though comparatively few animals are prepared to seek shelter from the cold, all of them have enemies against whom they must battle. These foes may wish to eat them or may simply wish to get them out of the way. In either event this struggle is so persistent and so keen that after starvation it is probably the source of the largest loss to the animal kingdom.

Considering first the feeding habits of animals, we find they are exceedingly varied. Some creatures simply engulf other and more minute animals, often only microscopic in size, in such quant.i.ties as to satisfy their hunger. Others, feeding upon larger plants or animals, must have some means of breaking off particles of this food; still others confine themselves entirely to nutritious fluids, and must have organs adapted to this particular type of food.

Insects are so common that anyone, who cares to, may easily verify what is here described. It will take nothing but a clear observant eye and a little patience to make out what is suggested. Each of our common insects has one of two clearly defined habits in the matter of food. Either it eats solid food, which must be made fine before it can be taken into the mouth, or it feeds upon liquids. These liquids may be easily accessible like the nectar of flowers, in which case one sort of mouth will serve; or they may be the juices inside the tissues of animals and plants, when an entirely different type of mouth must be employed in their acquisition. Perhaps the most easily found representative of the biting type of mouth, which breaks up solid food, will be seen in the common gra.s.shopper. Doubtless each one of my readers has at some time taken a gra.s.shopper into his hand, and, holding the tip of his finger against the insect's mouth, has promised the creature its freedom on condition that it disclosed its reprehensible habit of chewing tobacco. The gra.s.shopper surely complied, and I trust the promiser was as good as his word. The gra.s.shopper's head is so placed that, while it is at the front of its body, the mouth is directly on the under side of its head, while the eyes are at the top of the front of its face. Under these circ.u.mstances it cannot see what is going into its mouth, and this makes an interesting variation of conditions to which it must adapt itself. The means by which it accomplishes this will be clearer if the mouth of the gra.s.shopper be compared with our own. Our lips are upper and lower, but the gra.s.shopper has a front lip and a hind one. The broad front lip is easily seen at the forward side of the mouth. Just behind it, serving the purpose of our teeth, is a pair of hard jaws with h.o.r.n.y tips upon them, which serve to break small pieces from its food. While our jaws and those of all other backboned animals work up and down, so that we may be said to have an upper and lower jaw, the gra.s.shopper and all of his insect, crab, or spider relations, which have jaws at all, have them right and left, and they work from side to side. Behind these harder mouth parts is found a pair of softer jaws, each of which has on it a little finger-like feeler. With this pair the insect holds its food while the hard jaws break it to pieces. The hind lip follows, and is also provided with short finger-like feelers.

The feelers on the hind lip and on the soft jaw are necessary because the eyes are so placed as not to be able to see what goes into the mouth, hence the insect must make up for the loss of sight by the addition of touch. The same type of mouth as the gra.s.shopper has will be found among the beetles. Here the males sometimes have the hard jaws so enormously enlarged that they are known as pinchers and have given to their owners the name of pinching bugs. All insects with such jaws as these use them for breaking up solid food.

A glimpse at the mouth of the b.u.t.terfly captured on an adjoining flower will show a most remarkable variation from that seen in the gra.s.shopper. Practically all of the mouth parts mentioned are present in this insect, and its early ancestors had their organs practically like those of the gra.s.shopper. Now they are so modified and united with each other as to be almost unrecognizable. The pair of soft jaws has become very much elongated, and they lock together in such a way as to enclose a hollow s.p.a.ce between them through which the creature can suck its fluid food. Not only have these soft jaws joined together, but, because they have become so much elongated when not in use, they must be coiled up like a watch spring and laid between two hairy lip-like processes which correspond in reality to the two finger-like feelers of the gra.s.shopper's hind lips.

The b.u.t.terfly, lighting upon the corolla of the flower, uncurls this long "tongue," and through its hollow center pumps up into its crop the nectar which the flower has stored in its base. When the b.u.t.terfly comes to get the nectar from the flower, it rubs upon its own hairy body pollen from the stamens of the flower and carries it to the pistil of the next flower of the same kind which it visits. Most of us have at some time sucked the nectar from the back of a torn honeysuckle blossom and approved the taste of the b.u.t.terfly in this matter. If the airy creature be watched as it lights upon a flower, it will not be difficult to see it uncurl this long tongue and probe the depths of the flower. If the b.u.t.terfly be taken in the hand and the tip of a pin inserted in the center of the coiled tongue, it can be uncoiled without the slightest harm to the b.u.t.terfly.

Insects which wish to use for their food the juices of other animals or of plants do not find them so easy to gather. In the mosquito most of the mouth parts are developed into slender pointed bristles wrapped in a hind lip. These bristles serve to puncture the skin of the creature attacked, while the curled lip serves as a tube through which the blood may be extracted.

If, while sitting on the porch on a warm summer evening, mosquitoes begin to annoy, let one of them at least serve to show his method of procedure before he is destroyed. Allow the creature to alight upon the back of your hand and slowly raise the arm until the eye looking at near range can see the head of the mosquito, which, by the way, is sure to be a female. Males in this species are entirely harmless. They never eat after they have grown up; that is, after they are truly mosquitoes. But the female is very a.s.siduous. Alternately raising and lowering her lancets from either side, she pierces, then saws, her way down through the flesh until she has buried her instruments in her victim and her head rests against her prey. Now a pumping motion of the abdomen will be apparent, and this continues its accordion-like action until it becomes more and more distended. The insect only gives up its task when the entire abdomen is swollen into a great red ball of blood. The mosquito will now slowly withdraw its instruments and retire from the scene, if permitted to do so. If there is any fear of annoyance from the bite, a drop of ammonia immediately applied will counteract any irritation which would have been produced by the saliva of the mosquito. The insect is not intentionally vicious in this procedure. It is simply gathering its own natural food, though this does not make it less annoying to us since we are its victims. The swelling produced after the bite is the result of the action of the saliva the mosquito injected into the wound. The opening through the tongue is so small that blood would readily clot inside the tube and prevent its further usefulness, did not the mosquito inject the secretion of its salivary glands into the wound. This acts upon the blood in such a way as to prevent its coagulation.

Anyone who thinks carefully can add numberless specializations for food getting. For instance, primitive mammals have little pointed teeth which fit them for feeding on insects. In each of the great order of mammals a special development of these teeth has occurred.

Among the rodents or gnawing animals the front teeth have become long and chisel-shaped for nibbling. The horse has formed them for nipping, and his hind teeth for grinding. In the dog the teeth near the front have become long for tearing his flesh food, while his hind teeth, working with the motion of scissors, cut it into pieces.

A second great cla.s.s of specialization is seen in the changes of habit that provide the animal with shelter. The home seems so necessary a part of human life that it is almost impossible to think of an animal having nothing that in the faintest degree could be called a home. We at least expect it to have some sheltered place in which it pa.s.ses most of its time and to which it returns after its wanderings. The great majority of all animals have no such home. The place in which we find them to-day may not be the place in which they will be to-morrow.

All places are alike to them. The ordinary conduct of their daily life drives them about in the search for food. Their attempt to escape from their enemies leads them each day into new situations, and they may, and probably do, have no power to recognize the old location if they return to it. When we come to the backboned animals there is a little more tendency to a stationary location. The sun fish may frequent the same reach of the stream, the trout may haunt the same pool, year after year, but a great majority of fishes doubtless move indiscriminately up and down the stream or about the lake or ocean and are not found two successive days in the same place. The same may be said of frogs. For a time a particular frog may have a fondness for a special bend in the stream, but it is only a temporary fondness, I believe.

Our own need for shelter is the prime motive in leading us to build a home, and this necessity arises first of all because of our warm blood. What we are accustomed to call cold-blooded animals are not truly so. Their blood holds practically the temperature of their surroundings. As the air or the water in which they live grows warmer or colder the bodies of these creatures alter with it. Consequently they are active when the temperature is high and grow more sluggish as the thermometer falls. When the day grows distinctly cold the animals may go practically dormant.

Only the birds and mammals have warm blood, and of these the birds are distinctly the warmer. Whereas the temperature of the mammals runs from about ninety-eight to a hundred degrees Fahrenheit, that of birds lies somewhere between one hundred and five degrees and a hundred and ten. Creatures which are warmer than their surroundings must have some protection against chilling. Accordingly both mammals and birds have clothing. In the case of mammals the covering is fur, in the case of birds feathers. In some of the tropical animals like the elephant and rhinoceros, or in man, who has learned to protect himself in cold regions by making clothing for himself, this hair is very short, and except where serving for ornament is quite scanty, no longer being of use as a protection. But the great majority of all mammals are well covered with a dense coat of hair. In many of those living in the colder regions there is in reality a double coat. The fur seal of the Alaskan Islands is so provided. A set of long hairs deeply fastened in the skin forms a covering, which shows on looking at the seal. Underneath this layer, and set but lightly into the skin, is a short coat of very much finer hair known as the underpelt. When the skin is taken from the seal it is split by machinery into a lower and an upper layer. When so split the deep-seated pits of the long hairs are cut, and these hairs come out. The fine underpelt thus laid bare is what is commonly known as sealskin. Fas.h.i.+on has decreed that this must be dyed a rich brown, although when taken from the animal it is nearly mouse gray.

The birds have need for better clothing. To begin with, their blood is much warmer, and hence needs better protection from outside cold. In addition such of them as fly high must be prepared to stand great variations in temperature. For these purposes birds need a covering of the finest type. This clothing, in addition, must be extremely light because the creature must carry it into the air in flight. All of the requisite conditions are thoroughly met by the feather, which is the lightest and warmest clothing known to man. If at night we wish, regardless of expense, to keep ourselves warm with the lightest and warmest of covering, we send to the Arctic Sea, and from the breast of the eider duck we pluck the down which lies between the warm blood of the duck with its temperature of one hundred and seven degrees and the water in which the iceberg floats.

Young mammals and birds, before their clothing has well formed, are naturally susceptible to cold; this leads to the first genuine approach to a home among animals lower than man. Birds lay their eggs long before the creatures inside of them are ready to emerge.

Accordingly they have learned to build nests in which to place these eggs, and to protect them from the outside air; meanwhile the bird keeps the eggs warm by close contact with its own body. The lowest of the birds may lay their eggs simply on the ground without any special protection. As we rise in the scale of the bird world we find nests provided for the eggs. These nests become increasingly complex and specialized, until we reach the oriole's home with its wonderfully woven ma.s.s of fiber, which, in spite of its apparent looseness, supports well the weight of the mother bird and of her eggs. The robin, not content with making a woven basket, plasters it with clay, and makes an absolutely impervious nest.

When we remember that both mammals and birds are the modern descendants of cold and scaly reptiles of an earlier geological time, it becomes interesting to compare their clothing. Evidently in the mammals hairs began to come out between the scales. Gradually the scales became fewer and the hairs more abundant until finally the scales have all disappeared, except those that remain as the claws on the toes. The ancestors of the birds, on the other hand, boldly transformed their scales into feathers.

Another need for shelter arises in connection with the approach of winter. This problem of withstanding the cold season is complicated by the presence of two new factors. First and most directly, the cold itself is a distinct obstacle to the comfort of many of these creatures; as a secondary result of this cold, the food of many animals disappears entirely in winter. Most of our birds meet this difficulty by changing their base of operations. When the north grows cold these creatures fly to the south. Some of their migrations cover enormous stretches of country. Our bobolink, so well known and loved by all watchers of spring migrations, pa.s.ses twice a year between the lat.i.tude of New York and Rio Janeiro. One of our most careful students of bird migration says that the Golden Plover makes, twice each year, the long journey from the Arctic sh.o.r.es of North America to the plains of La Plata.

Different fur-covered animals have specialized to meet the winter by any one of three different methods. They may brave it out, hunting for their food as best they can all winter long. Such a course is pursued by the rabbit. Again like the squirrel, they may store large quant.i.ties of food during the summer, and on this provender they may subsist during winter, remaining for most of the time near their hiding-places, which, however, they may frequently leave upon warm days. A third method is less common, but very interesting. The groundhog or woodchuck is the best-known example of the group. It remains asleep, or, as it is technically known, dormant, during the winter. This stupor is more profound than ordinary sleep, and from it these animals awaken with difficulty. It is needless to remark that the groundhog's behavior on the second of February has no relation whatever to the weather we are to have later in the season. This is coming to be pretty generally understood. While the newspapers each year comment upon the groundhog and his shadow upon that day, year by year the notice has more of humor in it, and fewer people pay any attention to it.

As for the backboned animals which are cold-blooded, these must, unless they are fish, give up the struggle completely, bury themselves in out-of-the-way places, and go worse than dormant. They often become absolutely cold and stiff. In the case at least of fish, it is quite possible for them to be frozen stiff, even to be enclosed in cakes of ice, and still to recover if the encas.e.m.e.nt is not too long continued.

But the snakes, the turtles, the toads, the lizards, all are hidden beneath the ground waiting in absolutely unconscious rest the return of warmer weather.

After the need for food and shelter comes the continually recurring necessity on the part of almost every type of animal to escape from the unwearying persecution of higher creatures which would feed upon it. The whole creation is a constant network of animals which prey upon each other. It is the fate of a great majority of all creatures to fall victim to other animals to whom they serve as food.

Accordingly nature has concocted many devices by which she a.s.sists her favored children in escaping this relentless persecution. Perhaps the most widespread means which animals have developed in order to elude their enemies lies in the possession of power to escape their attention. Two different factors may contribute to this end. The first of these consists in the practice on the part of many animals of remaining absolutely quiet in time of danger. This instinct seems to be nearly universal. The first impulse of most animals upon discovering danger is to remain absolutely motionless. The eye detects, with ease, objects in motion. These same objects might entirely escape attention were they quiet. A mouse could remain in the corner of a room for a long time without attracting the eyes of the occupants of the room. Let it but scamper across the corner, and at once it is discovered. It is quite conceivable that early animals were divided in the matter; that the impulse of some was to escape from danger, while others, frightened by the presence of the enemy, remained absolutely still. Each plan has succeeded. Those which, on running, ran fast enough to escape became the parents of others like themselves, led eventually to a line of animals in whose speed lay their safety. Those, however, which attempted to escape, and failed because they were not swift enough, had their line cut off, and were thus less likely to be represented in the following generation. The constant result of errors along this line would be to destroy the slow and preserve the swift, and in the course of time it is quite thinkable that only the swift should remain. As the movements grew more and more keen, even the slower of these would pa.s.s out, thus tending always to produce the succeeding generation from those who were most rapid, and hence most likely to transfer to their children a similar power.

But there is another tendency of animals which leads them when frightened by their enemies to remain quiet. If this impulse is obeyed thoroughly enough, it is easy to see how the owner of this habit might entirely escape detection by his enemy. Any restless animal unable to restrain his nervous agitation naturally betrays his presence and is picked off. The result of evolution along this line would be the exact reverse of the preceding. Those that lay most absolutely quiet would be the parents of succeeding generations, while those who were slow in coming to rest, or were indifferent about remaining quiet, were picked off, and their tendency eliminated from the future of the species. In this way many animals have come to keep entirely quiet in the presence of danger. It is not a sign of high intelligence. As a matter of fact, it is rather a stupid procedure, so far as the animal itself is concerned, but it is a preserving stupidity, and many animals have it.

The "June Bug" (which is not a bug, but a beetle, and arrives in May) has this interesting habit of keeping quiet. If in its flight it strikes the globe of an electric light, it falls at once to the ground, and remains perfectly quiet for a time. After a short interval it recovers and starts out to regain its previous activity. But this recovery is by slow stages, and the whole procedure on its part looks exceedingly stupid.

The little snake with flattened and expanded head, known as the blowing viper, or puff adder, is one of the most amusing representatives of the tendency to "play dead" that could well be found. If you strike him the faintest blow with the lightest stick, he at once goes into apparent convulsions, in which he seems to suffer the greatest agony. Then, throwing himself upon his back, he, to all appearances, yields up the ghost. If, however, you retire but a slight distance and keep your eye upon him, you find that his ghost returns after a comparatively short absence, and he slinks away out of danger.

This is the most effective exhibition of this kind with which I am acquainted.

As for the habit of "playing 'possum" on the part of our opossum, the trick would seem to be particularly inane. The truth of the matter is, what is attributed to an unusual brilliancy on the part of the creature is positively unusual witlessness. The animal has an exceedingly small brain, as compared with that of a dog of similar size, and to anyone who knows brains at all this particular organ would not be looked upon as furnis.h.i.+ng its owner much ability. The fact is that the opossum has exceedingly small wit, and this little deserts it in an emergency, as a result of which he grows helpless and motionless. This is often supposed to indicate great wisdom. There may be wisdom in it, but it is the wisdom that lies back of all nature. It certainly is not the wisdom of the opossum.

Man himself possesses to a marked degree this impulse to keep quiet in danger. The man from the country who is visiting the large city, suddenly startled by the "honk" of the auto horn, finds his power of movement promptly arrested, and he is not unlikely to be struck and injured by the machine from which the city dweller would easily escape. This is not particularly to the credit of the city dweller, who, when in the country, may find himself similarly startled by the sudden appearance of the calf, the pig, or the sheep. The bull, which a country boy, accustomed to him from childhood, will drive with a willow switch, is a source of terrified concern to the city man.

While the trick of keeping quiet serves many an animal in time of danger, there is another device for escaping attention, far more common and widespread throughout the animal world. The eye does not easily see an object if it is colored like the background against which it stands. A host of animals find their main safety in being indistinguishable in color from the surface on which they live. There are many biologists who seriously question whether protective coloration, as Darwin called it, is as effective as he believed it. In some quarters it is the present fas.h.i.+on to doubt protective coloration entirely. No one has yet shown any principles which will better explain the great color scheme of the animal world, and until such explanation is forthcoming I believe it will not be wise for us to discard the idea of protective coloration. No doubt it has been overworked by enthusiastic believers in its efficiency. At the same time, to overlook it completely, is, I believe, to make a greater error. I have little doubt that when the broader explanation comes, which will satisfactorily explain the color scheme of the animal world, the idea of protective coloration will be found, not so much to have been wrong, as to have been but partial. It will be included under the broader principle which takes its place and will not be supplanted by it.

The idea of protective coloration is that very many animals have ordinarily come to be colored like the background on which they live.

The process has taken many generations, and is very slow, but is none the less sure in the end. In most cases the animal is probably entirely unconscious of this point in its favor, and usually it does nothing to a.s.sist the deception. The result is none the less effective because the animals themselves are unconscious of the process. The cabbage worm is green in color like the cabbage. This does not mean that it got green by eating cabbage or by longing for greennesses.

Through long years the enemies of the cabbage worm have been picking it off the plants on which it fed. This does not imply that cabbages as we know them are very old, but cabbage worms doubtless ate the leaves of the sea-kale long before man had cultivated it into cabbage.

During all these years the enemies of the caterpillars, generally in the shape of birds, have been a.s.siduously gathering them up.

When we see how much the various members of the same human family may differ in complexion, how much the various pigs in the same litter may differ in size and in coloration, it is easy to understand that among these caterpillars which have eaten the cabbage there must have been considerable color variations. I do not imagine for a moment that the birds had any preference for any particular color in their cabbage worms. They took every caterpillar they saw, but they naturally first saw those that were least like the background on which they lived.

The only caterpillar which was effectively hidden from his enemy was the one that was indistinguishable on the leaf. If it escaped in this way, the probabilities are that it would produce young which would be at least a little more likely to be green in color than the progeny of its darker-colored brothers and sisters. By this continued process the birds steadily weed out the darker-colored specimens. There would result, in the course of time, a race of caterpillars, whose ancestors for so many generations back had been light green in color, that there is little likelihood of any of the older and darker forms turning up again. In the course of time all dark tendencies will have disappeared from the family and practically all of the group will be light green.

Any sport or variation in the shape of greater conspicuousness would fall a quick prey to the enemy and its line be cut off forever.

The same sort of activity has resulted in the peculiar green color of the katydid. This creature lives chiefly upon the leaves of trees and shrubs. This insect is so large that, even though it is leaflike in color, it might still be conspicuous. As a result those katydids whose wings were most like leaves in form were least likely to be picked up by the pa.s.sing bird. This sort of protective appearance is intensified by exactly the same means as that which brought about protective coloration. The katydid least leaflike in appearance was eaten first.

Thus those most leaflike remain until the last, and are most likely to produce young. Again, it was not the fact that they lived among leaves which made them look leaflike, but it is because they look like leaves that they escaped being devoured.

The katydid has materially a.s.sisted in its own preservation by being active chiefly at night. In the daytime it keeps comparatively quiet.

Thus seated upon a twig, especially if hidden among the leaves, it is almost unnoticeable. At night, however, it moves about more freely, seeking its food and eventually its mate. At such times it becomes distinctly more conspicuous because its wings are steadily fluttering.

The hind wings are filmy and are very light green. The creature looks most ghost-like as it flies through the evening air.

A very similar history lies back of the coloring of the ordinary toad.

Though descended from the frog, and originally a creature of the water, the toad has long since adapted itself to live upon the dry ground. It still produces its young in the water as it did when a frog. Whereas the childhood of the frog, that is, its tadpole stage, is very long and it a.s.sumes its adult form comparatively late, just the reverse is the case of the toad. The young hasten through their tadpole stage within a few weeks, and a.s.sume the shape of the parent toad when about big enough to cover your little fingernail. Now they leave the water and seek dry land. Naturally they make the change when the land is damp, that is, after a warm spring rain. People seeing these mult.i.tudes of little toads hopping around over a bare spot of ground, and remembering the rain of the night before, insist that it has rained toads. Of course it never rains down anything which cannot evaporate up. The stories of showers of toads and of earth worms, with an occasional fish, or even creatures of larger size, are all pure myths. There are conceivable tornadoes after which there might be a shower of such creatures, but at such a time it is likely also to rain barn roofs and buggies. You may be sure that toads which come down in the rain are dead after they strike the ground.

The little toads started out, perhaps a hundred at a time, from the small pool in which their eggs were laid. These creatures find dragons on every side. The gartersnake comes along and gets his first toll; the heron follows him and takes such as catch his hungry eye; the turkey gobbles up his from what are left. By the time the toad-eating creatures in the neighborhood have taken such as they found, there are very few remaining. These doubtless have been left for a very good reason, generally because they were not noticed. This was because they looked like the ground on which they sat, and because they kept perfectly quiet while the enemy moved about. This process has gone on so long that the toad has come to be astonis.h.i.+ngly well protected by its resemblance to the ground. This likeness it intensifies by its interesting habit not only of keeping entirely quiet, but of dropping its nose to the ground, instead of sitting high on its front legs, as it does when not in danger.

I have noticed that if a snake and a toad be placed in the same cage, when the snake approaches to capture the toad the toad drops into a squatting position, and is very likely to blow himself up until he is rounder in outline than he was before. Whether this is a deceptive trick which makes him the more resemble a stone is more than I can say. I do not remember having seen our eastern toad do it. I have seen it happen a number of times in the laboratory of a Colorado naturalist, and it is quite possible that in the open country more spa.r.s.ely covered with vegetation than is our ground in the east this inflating device may serve the toad more effectually than if it kept its own outline.

Even among creatures far more active than the toad and the katydid an inconspicuous color must certainly result in distinctly better protection. Everyone knows the jay and the cardinal when first he has seen them, if only he has a slight acquaintance with their pictures.

They are so conspicuous that we recognize them at once. More common in my region than the jay or the cardinal is the red-eyed vireo. This creature moves industriously in and out among the leaves of our trees.

It is persistently in motion, is nearly constant in song, and is a bird of fair size, being larger than our English sparrow, though smaller than a robin. Many a nature lover will recognize twenty-five or thirty birds at sight without any difficulty, and not know the vireo. Yet the vireo is more common than two-thirds of the birds he knows. There can be but one reason for this; the bird is inconspicuous. The olive-green of its back, with its light under parts, serves to hide it completely amid the foliage. Even the bird-lover learns to find it first by its jerky song, and then by watching for its movements among the leaves.

One aspect of protective coloration has been brought to our attention by the artist, Mr. Abbott N. Thayer. He first clearly explained why it is that animals are usually so much lighter on the under side than they are upon the upper. Mr. Thayer proves his position by taking some ordinary cobblestones and painting one of them a uniform color and placing it upon a board painted the same color. One would think the stone would be inconspicuous; as a matter of fact, is quite easily seen. The underside of the stone, turned away from the light, is so shaded as to mark a distinct boundary between the stone and the board.

Another cobblestone was colored on its upper side like the board, but the color faded into a lighter and lighter tint until the bottom of the stone was nearly white. This stone, placed upon the board, was at a short distance nearly invisible. In other words, although the pigment was actually lighter on the under side, it was so much less intensely illuminated, that the result was the same in tint as the other side under the clear sharp light of the sky.

Many a person, looking down into the water from a bridge, sees nothing whatever of the fish in the water below, because their backs are exactly like the bottom of the stream. Suddenly one of the fish, by a quick movement, turns its lighter under side over in such a way that it is clearly illuminated from the sky. Immediately a flash as of silver strikes the eye of the onlooker and makes him aware of the presence of the fish which had previously been undetected. If rendered thus suspicious, the observer will carefully examine the bottom of the water, he may quite likely find dozens of fish which had previously escaped his attention.

Nature is very versatile. So many of her apparently chance ventures have proved successful that she has retained many devices by which her children may be safe. One of these, which is doubtless often quite effective and may serve to save an animal's life, is that of being able to emit an odor so nauseating as to offend the enemy's sense of smell, and doubtless remove the keen edge of his appet.i.te. It is not uncommon among the group of insects properly known as bugs to possess an exceedingly unpleasant odor. Anyone who has handled a squash bug will know exactly what I mean, and there are other members of the group not so common as the squash bug, which, at least to the human nose, are distinctly offensive. Some of the beetles also save themselves by this device.

One of the most interesting developments of this peculiarity is found in the case of the common skunk. This creature has in each groin a gland capable of secreting a highly offensive fluid. Ordinarily this liquid is kept safely within its sac, and for a long time none of it may escape. When closely cornered, the skunk will turn its tail toward the enemy and with a quiver and a flip of his tail it can guide the openings of two little tubes that come out along the root of the tail in such fas.h.i.+on as to eject the fluid in a fine and foul-smelling stream against the animal from which the skunk would escape. Once fairly hit by this fluid, I imagine most animals will drop the skunk.

A dog surely will, and will hate himself for having made the attempt to capture anything which must be so ignominiously allowed to escape.

If ones clothing is well saturated with it, it is nearly useless to hope to remove the odor. A dog will carry the smell for several weeks.

For a long time it will be so strong as to make him an unfit denizen of the house. Even swimming in deep water does not remove it. After two weeks, although he may seem to be practically free from the odor, a light rain will bring it all out again and make him nearly as offensive as before.

Not as prompt in its action, but in the end nearly as effective, is the protective device which the toad sometimes uses to his distinct advantage. May I be pardoned a personal account of this particular feature. It was my good fortune to be for a short time a student in a cla.s.s taught by Edward Drinker Cope, one of the most brilliant of our American biologists. Prof. Cope mentioned in cla.s.s the fact that the Batrachians (the group to which the toad belongs) have in many cases the power to emit from their skin a fluid which is sufficiently nauseous to deter an animal from eating the creature that secretes it.

Upon such authority as this, I had no hesitancy whatever in repeating Cope's statement. One morning I had a cla.s.s in the field studying the ground ivy, whose dainty blue flowers were lifting themselves out of the dewy gra.s.s. While we were thus engaged, a toad joined the circle.

He came out of his dewy retreat clean and fresh from his morning bath.