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 appetite. 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 fashion 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 class taught by Edward Drinker Cope, one of the most brilliant of our American biologists. Prof. Cope mentioned in class 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 class in the field studying the ground ivy, whose dainty blue flowers were lifting themselves out of the dewy grass. While we were thus engaged, a toad joined the circle. He came out of his dewy retreat clean and fresh from his morning bath. I took him in my hands, and made him the subject of an immediate lesson. I showed to my pupils his eyes and his interesting method of handling them, his tongue and its strange insertion; showed them how to look into his mouth and look up his ears to his ear drums, and pointed out many other interesting facts. Then I told them how Cope had said that the toad had power to emit from its skin a fluid so nauseous that many an animal hesitates to eat it. This is the first peculiarity I had mentioned which I had not myself observed, and a scientific qualm came over my conscience. Why had I never verified this statement which I had so frequently repeated? On the impulse of the moment, with the bright, clean skin of the creature fresh from the dewy grass, making it less than usually repulsive, I ran my tongue up its back only to find that it had no taste whatever. I was of course surprised, but I was not foolish enough to deny, as the result of one observation, the statement of a good scientist. The observation, moreover, was one which I naturally did not care to repeat with any frequency. Of one thing I was sure, toads do not always have an unpleasant taste.
A year later I had a class down by the side of a neighboring pond. The pool was not an attractive one, and I had picked from it a more than commonly unappetizing looking toad, which proved to be a mother which had not yet laid her eggs. As I held her in my hands and exhibited her various points to my pupils, I told them of Prof. Cope's statement. I also told them of my unsuccessful attempt the previous year to verify the statement. I added, however, that I would not repeat this experiment on this unappetizing specimen. Hereupon the toad not only exuded, but squirted, from a gland over her left shoulder blade a fluid, milky-like in appearance, and forming a jet as thin as a needle, but ejected with force enough to strike my face, which was at least fifteen inches away. I moistened my finger on my tongue, lifted the fluid from my cheek, and tasted it. Cope was right. A toad can exude a most nauseous fluid. Horsechestnuts extracted and distilled might possibly provide something as bitter. Why did I not find this in the preceding case? I have too few observations on which to base a conclusion, but I have a suspicion as to the reason. In the case of the toad which spurted the fluid in my face, we had a creature with whose life were tied up the lives of her many offspring, to be produced from the eggs she was so soon to lay. Under conditions like these, nature is more than commonly careful of her children. Whether this be the reason or not, toads do not always have an unpleasant taste, but when they do it certainly is most unpleasant.
There remains to be considered the most effective plan yet mentioned of escaping the enemy, and that is of really escaping. In all the devices we have considered thus far the enemy is eluded. When the creature lies quiet, or finds safety in its protective coloration, or in its bad taste, or unpleasant odor, it still remains in the presence of the enemy. A more progressive plan altogether is to escape the enemy by flight. The great advantage of this plan lies in the fact that the acquisition is valuable for every purpose. The creature then can escape the enemy, can range widely for food or for a mate. This gives it an enormous advantage in the struggle for life. The power to fly, in insects, was doubtless originally gained in the attempt to escape the enemy. Among many of the lower animals it is nearly the only purpose that flying serves. Later on it enables the animal to pass from one food locality to another. In a few creatures it plays an effective part during the mating season. These last are probably both derived powers, and the original function was that of escape from the enemy. The grasshopper has grown its long legs to serve him for safety, and through them it is helped along, moving about chiefly by leaps when it wishes to go any material distance. It is only toward the very end of its life that the grasshopper has wings, and then they serve probably to aid in the search for a mate. Among the birds flight began simply in sailing out of the trees, into which the creature, still half lizard, had crept to escape its enemy. The earliest bird known to us had comparatively insignificant wings. There was really more support in its tail than in its wings, and this would distinctly indicate that it glided more than it flew. It had claws also upon its wings, and it was probably the case that this creature crept into the trees, at least in its earliest forms, and sailed down in a manner not unlike that employed to-day by the flying squirrel. From such simple beginnings came the wonderful power of flight in the birds.
Among mammals the attempt to escape from the enemy has led to an interesting development, which will be more fully explained in a later section when we speak of the history of the horse. The early mammals walked flat-footed, as we do on our feet and as the raccoon and the bear do on theirs. Gradually, however, as their enemies became more fierce and better able to injure the larger mammals, the latter gained in power of flight, and this gain consisted first in rising from the toes, lifting the heels completely off the ground. At the same time the leg and foot were gradually lengthened. Doubtless in this way the fleet animals, like the deer, the horse and the giraffe, first came by their long legs. Constant elimination of the short-legged ones, by the pursuing enemy, resulted in the selection of the long-limbed ones for breeding purposes, and hence to the ultimate elongation of the legs of the species.
The method of escape from the enemy involves cowardice. "He who fights and runs away may live to fight another day," and so it may be the part of wisdom in the weak creature to escape from his enemy by flight. It is a far more estimable process, from our standpoint at least, to stand against the onslaught of the enemy and beat him upon his own ground. This end is secured in many animals by acquiring horns or by lengthening certain of the teeth. The horn is a very ancient instrument of defense. When the reptiles ruled the land horns were not uncommon. They consisted in those days of hardened scales, which lengthened and fastened themselves over a core of bone. Such an old-fashioned instrument, sometimes made of newer materials, still remains the defense of a number of animals. The rhinoceros has upon his nose a lengthened projection, which is what might not improperly be called hair glued into a cone. This enormous horn is a frightful weapon, both of offense and defense, and, when backed by the terrible weight of the body of the rhinoceros, it can do as deadly work as almost any instrument of destruction known to animals below the grade of man. But, after all, this is an old-fashioned method, and the rhinoceros is a relic.