( Originally Published 1909 )
ALTHO but one element of organic evolution, the origin of species, has been emphasized in the preceding pages, it has been noticed perhaps in the various discussions brought up that evolution is concerned not only with the great variety of life kinds, but also with the "adaptedness or adaptiveness of life kinds" to various sorts of life conditions. As Kellogg phrases it, "The task of an evolution explanation is a double one ; it must explain not only diversity or variety in life, but adaptive diversity or variety."
The most striking fact in nature is this adaptation of organisms to their environment. On every side plants and animals seem to be well fitted to their particular places in nature. Every organism seems to have been constructed after an ideal plan, and it is not surprising that so many observers of nature have believed that plants and animals were specially designed and created to fit the places in nature which they fill. Verworn says:
"The fact of purposefulness in living nature, which was so marvelous to men of science in early times even down to the middle of the present century, forced them constantly to embrace teleology i.e., the hypothesis of a fore-ordained plan of creation, such as dogmatic theology, preserving faithfully the ancient venerated ideas, accepts to-day. This purposefulness in nature is the simple expression or, better, the result of the adaptation of organisms to their vital conditions in the widest sense."
Thus all animals have their essential organs adapted to habitat, food and various conditions of environment. Fishes have limbs in the form of fins, which function very perfectly as rowing-organs; terrestrial vertebrates have in place of fins legs for walking and creeping upon dry land; birds have wings constructed most fittingly with which their light bodies, supported by bones containing air, soar through the air so perfectly that imitation of them is difficult.
Likewise plants are so perfectly adapted to their surroundings in the general plan and arrangement of root, stem, leaf, flower, etc., that only the special student is likely to recognise the adaptation. In fact, these general adaptations of organs and functions of plants and animals are so universal that they have ceased to excite wonder and are taken as necessary phenomena of life. They are necessary phenomena of life, for general adaptations are as natural as breathing, eating and so on.
In addition to the general adaptations to widespread conditions of environment e.g., to the general conditions under which all land animals and plants or all aquatic animals and plants must live there are myriads of special adaptations. For example, all fishes living in fresh water have in common general adaptations to an aquatic life, but fishes living in caverns have in addition to the general adaptations certain special adaptive modifications in accordance with their special environment, especially in the eyes, which are profoundly modified in accordance with a life spent habitually in darkness. Likewise all plants living on land have a general adaptation to terrestrial life, but in addition many have special adaptations. All plants which have green leaves use them for starch formation under the action of sunlight, but special adaptations fit certain species to different degrees of light intensity, and as a result one plant may be fitted to grow in open fields while another will grow but in shaded places.
Special adaptations have attracted much attention from students because by contrast with general adaptations they prove valuable in evolutionary studies. For the sake of convenience the special adaptations of plants and animals will be considered separately, and also color adaptations will be treated in a separate section. However, this is purely an arbitrary division, for all studies of the special adaptations of plants and animals lead to the same conelision namely, that these adaptations have originated through the processes of evolution.
One of the best collections of facts concerning special adaptations is to be found in the extended chapter on this subject in Jordan and Kellog's 'Evolution and Animal Life,' from which some of the most striking examples have been selected for use here.
The various types of special adaptations may be roughly divided into five classes as follows : Food-securing, self-defense, defense of young, rivalry and adjustment to surroundings.
"For the purpose of capture of their prey, most carnivorous animals are provided with strong claws, sharp teeth, hooked beaks and other structures familiar to us in the lion, tiger, dog, cat, owl and eagle. Insect-eating mammals have contrivances especially adapted for the catching of insects. The ant-eater, for example, has a long sticky tongue which it thrusts forth from its cylindrical snout deep into the recesses of the ant-hill, bringing it out with its surface covered with ants. Animals which feed on nuts are fitted with strong teeth or beaks for cracking them. Strong teeth are found in those fishes which feed on crabs or sea urchins. Those mammals like the horse and cow, that feed on plants, have usually broad chisel-like incisor teeth for cutting off the foliage, and teeth of very similar form are developed in different groups of plant-eating fishes. Molar teeth are found when it is necessary that the food should be crushed or chewed, and the sharp canine teeth go with a flesh diet. The long neck of the giraffe enables it to browse on the foliage of trees in grassless regions.
"Insects like the leaf-beetles and the grasshoppers, that feed on the foliage of plants, have a pair of jaws, broad but sharply edged, for cutting off bits of leaves and stems. Those which take only liquid food, as the butterflies and sucking bugs, have their mouth parts modified to form a slender, hollow sucking beak or proboscis, which can be thrust into a flower nectary or into the green tissues of plants or the flesh of animals to suck up nectar or plant sap or blood, according to the special food habits of the insect. The honey-bee has a very complicated equipment of mouth parts fitted for taking either solid food like pollen or liquid food like the nectar of flowers. The mosquito has a 'bill' composed of six sharp, slender needles for piercing and lacerating the flesh and a long tubular under lip through which the blood can flow into the mouth. Some predaceous insects, as the praying horse, have their fore legs developed into formidable grasping organs for seizing and holding their prey.
"For self-protection the higher animals depend largely on the same organs and instincts as for the securing of food. Carnivorous beasts use tooth and claw in their own defense, as well as in securing their prey, but these, as well as other animals, may protect themselves in other fashions. Many of the higher animals are provided with horns, structures useless in procuring food but effective as weapons of defense. Others defend themselves by blows with their strong hoofs. Among the reptiles and fishes and even among the mammals the defensive coat of mail is found in great variety. The crab and lobster, with claws and carapace, are well defended against their enemies, and the hermit crab, with its well knowl habitude of thrusting its unprotected body within a cast off shell of a sea snail, finds in this instinct a perfect defense. Insects also, especially beetles, are protected by their coats of mail. Scales and spines of many sorts serve to defend the bodies of reptiles and fishes, while feathers protect the bodies of birds and hair those of most mammals."
The ways in which animals make themselves disagreeable or dangerous to their captors are almost as varied as the animals themselves. Besides the teeth, claws and horns of ordinary attack and defense, there are found among the mammals many special structures or contrivances which serve for defense through making their possessors unpleasant.
The turtles are all protected by bony shields, and some of them, the box turtles, may close their shields almost hermetically. The snakes broaden their heads, swell their necks or show their forked tongues to frighten their enemies. Some of them are further armed with fangs connected with a venom gland, so that to most animals their bite is deadly.
Even the fishes have many modes of self-defense through giving pain or injury to animals who would swallow them. The catfish, or horned pout, when attacked sets immovably the sharp spine of the pectoral fin, inflicting a jagged wound. Pelicans which have swallowed a catfish have been known to die of the wounds inflicted by the fish's spine. In the group of scorpion fishes and toad fishes are certain genera in which these spines are provided with poison glands. Many fishes are defended by a coat of mail or a coat of sharp spines. The globe fishes and porcupine fishes are for the most part defended by spines, but their instinct to swallow air gives them an additional safeguard. When one of these fishes is disturbed it rises to the surface, gulps air until its capacious stomach is filled and then floats, belly upward, on the water. It is thus protected from other fishes, tho easily taken by man.
The torpedo, electric eel, electric catfish and star-gazer surprise and stagger their captors by means of electric shocks. The shock is felt severely if the fish be stabbed with a knife or metallic spear. The electric eel of the rivers of Paraguay and southern Brazil is said to give severe shocks to herds of wild horses driven through the streams, and similar accounts are given of the electric cat-fish of the Nile. In tropical seas the tangs, or surgeon fishes, are provided with a knifelike spine on the side of the tail, the sharp edge directed forward and slipping into a sheath. This is a formidable weapon when the fish is alive.
Other fishes defend themselves by spears (swordfish, spearfish, sailfish), or by saws (sawfish, sawshark), or by paddles (paddlefish). Others still make use of sucking disks of one sort or another (as in the snailfish, the cling fish and the goby) to cling to the under side of rocks, or as in the Remora, to the bodies of swift-moving sharks. Blind fishes in the caves are adapted to their condition. In similar circumstances salamanders, crayfishes and in-sects are also blind. Some fishes, as the lancelet, lie buried in the sand all their lives. Others, as the sand darter arid the hinalea, bury themselves in the sand at intervals to escape from their enemies. Some fishes called the flying fishes sail through the air with a grasshopper-like motion that closely imitates true flight.
Among the insects the possession of stings is not uncommon. The wasps and bees are familiar examples of stinging insects, but many other kinds less familiar are similarly protected. All insects have their bodies covered with a coat of armor, composed of a horny substance called chitin. In some cases this chitinous coat is very thick and serves to protect them effectually. This is especially true of the beetles. Some insects are inedible and are conspicuously colored so as to be readily recognised by insectivorous birds.
The protection of the young is the source of many adaptive structures as well as of the instincts by which such structures are utilized. In general those animals are highest in development, with the best means of holding their own in the struggle for life, that take best care of their young. Those instincts which lead to home building are all adaptations for preserving the young. Among the lower or more coarsely organized birds, such as the chicken, the duck and the auk, as with the reptiles, the young animal is hatched with well-developed muscular system and sense organs and is capable of running about and, to some extent, of feeding itself. Birds of this type are known as praecocial, while the name altricial is applied to the more highly organized forms, such as the thrushes, doves and song birds generally. With these the young are hatched in a wholly helpless condition, with ineffective muscles, deficient senses and dependent wholly upon the parent. The altricial condition demands the building of a nest, the establishment of a home and the continued care of one or both of the parents. In the Marsupials-the kangaroo, opossum, etc. the young are born in a very immature state and are at once seized by the mother and thrust into a pouch or fold of skin along the abdomen, where they are kept until they are able to take care of themselves. This is a singular adaptation, but less specialized and less perfect than the condition found in ordinary mammals.
The movements of migratory fishes are mainly con-trolled by the impulse of reproduction. Many fresh-water fishes, as trout and suckers, forsake the large streams in the spring, ascending the small brooks where their young can be reared in greater safety. Still others, known as anadromous fishes, feed and mature in the sea, but ascend the rivers as the impulse of reproduction grows strong. Among such fishes are the salmon, shad, alewife, sturgeon and striped bass in American waters. Catadromous fishes, as the true eel, reverse this order, feeding in the rivers and brackish estuaries, apparently finding their usual spawning ground in the sea.
In questions of attack and defense the need of fighting animals of their own kind, as well as animals of other races, must be considered. To struggles of species with those of their own kind the term rivalry may be applied. Actual warfare is confined mainly to males in the breeding season, especially in polygamous species. Among those in which the male mates with many females, he must struggle with other males for their possession. The most notable adaptation is seen in the superior size of teeth, horns, mane or spurs. In the family of deer, buffalo and domestic sheep and cattle the male is larger and more powerfully armed than the female.
A large part of the life of the animal is a struggle with the environment itself. In this struggle only these that are adapted live and leave descendants fitted like themselves. The fur of mammals fits them to their surroundings. As the fur differs so may the habits change. Some animals are active in winter; others, as the bear, and in northern Japan the red faced monkey, hibernate, sleeping in caves or hollow trees or in burrows until conditions are favorable for their activity. Most snakes and lizards hibernate in cold weather. Some animals in hibernation may be frozen alive without apparent injury. As animals resist heat and cold by adaptations of structure and habits, so may they resist dryness. Certain fishes hold reservoirs ,of water above their gills by means of which they can breathe during short excursions from the water.
Another series of adaptations is concerned with the places chosen by animals for their homes. The fishes that live in the water have special organs for breathing under water. The hooked claws of the bat hold on to rocks, the bricks of chimneys or to the surface of hollow trees, where the bat sleeps through the day. The tree frogs or tree toads have the tips of the toes swollen, forming little pads by which they cling to the bark of trees.
Among other adaptations relating to special surroundings or conditions of life are the great cheek pouches of the pocket gophers, which carry off the soil dug up by the large shovel-like feet when the gopher excavates its burrow. Insects that live in water either come up to the surface to breathe or take down air underneath their wings, or in some other way, or have gills for breathing the air which is mixed with the water. Many fishes, chiefly of the deep seas, develop organs for producing light. These are known as luminous organs, phosphorescent organs or photophores.
While aming the higher or vertebrate animals, especially the fishes and reptiles, most remarkable cases of adaptation occur, yet the structural changes are for the most part external, usually not affecting fundamentally the development of the internal organs other than the skeleton. The organization of these higher animals is much less plastic than that of the invertebrates. In general the higher the type the more persistent and unchangeable are those structures not immediately exposed to the influence of the struggle for existence. It is thus the outside of an animal that tells where its ancestors have lived. The inside, suffering little change whatever the surroundings, tells the real nature of the animal.
A special kind of adaptation is shown by animals which are parasitic. These animals attach themselves to the body of their prey or burrow into it, are carried about by it and live upon it. Some parasites are adapted to an external parasitic life, such as bird lice, fleas, ticks, etc. ; others are adapted to an internal parasitic life, as is the case in the tapeworm and trichina.
In nearly all cases the structure of the parasite is very simple, much simpler than that of other animals which live free, active lives. This simplicity, however, is not primitive, but results from the degeneration of structures rendered useless by the habit of life. Thus a fixed and permanent parasite possesses no locomotor organs, no special sense organs, no highly developed nervous system, no alimentary system and but a very simple circulatory and respiratory system. Lankester has well expressed the effects of the parasitic habit of life : "Let the parasitic life once be secured and away go legs, jaws, eyes and ears; the active, highly gifted crab, insect or annelid may become a mere sac, absorbing nourishment and laying eggs."
This simplicity of structure in parasitic animals does not, however, indicate that they belong to animals low in the scale of animal life. It is rather the result of a mode of life. This is shown by the fact that many parasites in their young stages are free, active animals, very much more complex in structure than they are in adult life. Lankester's comparison of the life history of the parasites Sacculina and Lernaeocera with that of an active shrimp will illustrate this point. Diagrams are given, showing the advancing complexity of structure in the shrimp Peneus.
Compare these with the young stages of a number of shrimp like animals, viz., Sacculina, Lernaeocera, Lepas, Cyclops, Limnetis, some of which lead a parasitic life. The eggs of all develop equally into the recapitulative phase known as the Nauplius, but while the Nauplius of the free living shrimp grows more and more elaborate, observe what happens to the parasites; they degenerate into comparatively simple bodies, and this is true of their internal structure as well as of their external appearance. The most utterly reduced of these parasites is the curious Sacculina, which infests hermit crabs and is a mere sac filled with eggs, and absorbing nourishment from the juices of its host by root-like processes.
Lernaeocera again, which in the adult condition is found attached to the gills of fishes, has lost the well developed legs of its Nauplius childhood and become an elongated worm like creature fitted only to suck in nourishment and carry eggs.
In this same group the life-history of the barnacle illustrates a similar degeneration not due to parasitism. This again Lankester describes. Among these pictured NaupIii, all belonging to the great group Crustacea, which includes crabs and shrimps, is one which gives rise to an animal decidedly degenerate but not precisely parasitic in its habits. The egg of the Barnacle gives rise to an actively swimming Nauplius, the history of which is very astonishing. After swimming about for a time the Barnacle's Nauplius fixes its head against a piece of wood and takes to a perfectly fixed, immobile state of life. Its organs of touch and of sight atrophy, its legs lose their locomotor function and are simply used for bringing floating particles to the orifice of the stomach, so that an eminent naturalist has compared one of these animals to a man standing on his head and kicking his food into his mouth.
Were it not for the recapitulative phases in the development of the Barnacle, we may doubt whether naturalists would ever have guessed that it was a degenerate Crustacean. It was, in fact, for a long time regarded as quite remote from them and placed among the snails and oysters.
Its true nature was only admitted when the young form was discovered.
Very many other organisms among both plants and animals showing varying degrees of degenerative adaptation might be cited. It should be noted in this connection that degeneration in biology means an evolutionary method of adaptation by means of which plants and animals are adjusted to special environmental conditions; it does not mean weakness, decline, defects and decay in structural and physiological conditions similar to those occurring in human life. There is only a far-fetched analogous resemblance between human degeneration in the usual sense and degeneration in biology which may be called adaptive because as a result of such degeneration organisms are better fitted for life under special conditions. In fact, many naturalists believe that natural selection has often preserved those individuals which because of certain de-generations are better fitted to their life conditions. Hence adaptive degeneration in biology is a form of evolution, and it is a highly significant fact that some species of animals and plants have been fitted to their special environment by adaptive degeneration.
It should be noted in passing that in animals and plants there is nonadaptive and destructive degeneration that is parallel to degeneration in human life. The effects of disease in weakening and ultimately destroying animals and plants is an example. In all such cases there is no advantage gained which is exactly the case in the physiological, mental and moral degeneration commonly referred to in human life. Obviously adaptive degeneration has occurred in human life, for the human body has dozens of structures like the appendix which have been adapted by a degenerative process.
Some of the most important special plant adaptations are along these lines : nutritive adaptations, adaptations for protection against animals, adaptations for pollination, adaptations for the dispersal of fruits and seeds, and color adaptations for attracting animals. Of the nutritive adaptations the parasitic habit of life in plants which draw their food supply wholly or partially from another plant or animal, called the host, should be mentioned. The dodder shows an adaptation of this kind. It lives on the stems of other plants, and instead of developing roots and green leaves with which to carry on the processes of food manufacture, it develops no green leaves but instead a special absorbing organ, the 'haustoria,' which penetrates the tissues of the host plant from which it obtains its nourishment. There are many plants adapted to this parasitic habit of life; others are semi-parasitic. This is the case with the mistletoe, the false foxgloves and with some orchids.
A curious case of special nutritive adaptation is to be found in carnivorous plants. These plants seem to require animal food and have organs variously modified to obtain it. In the common sundew insects are caught by a sticky secretion which proceeds from hairs on the leaves. When an insect touches one of these sticky hairs it is caught and the hairs at once begin to close over it until it is held fast on the leaf. Here it soon dies and then remains for many days, while the leaf pours out a juice by which some parts of the insect are digested. This digested material is then absorbed, while the undigested parts drop off after the hairs let go their hold. Other interesting adaptations for the capture by plants of insects are to be found in the Venus fly trap, the common pitcher plant and others.
Other modifications have been developed as a means of protection from vegetable-feeding creatures. These are chiefly along the lines of the formation of uneatable tissue, as in the horsetails and rushes, the arming of exposed parts with cutting edges, stinging hairs, prickles and thorns, as in grasses, nettles, cactuses, etc., and the accumulation of disagreeable or poisonous substances in exposed parts, as in the tansy, ragweed, boneset, jimson weed and poisonous hemlock. One of the acacias (Acacia sphaerocephala) has an interesting adaptation to attract ant dwellers as a protection from insects and other creatures. At the bases of the leaves there are developed large hollow stipules and at the ends of the leaflets are nectaries. The ants bore holes in the stipules, live in them, find food in the nectaries and offer valuable protection to the plant in warding off its enemies. A large number of plants offer inducements of many sorts to attract ant visitors.
The special adaptations for the dispersal of seeds offer an equally interesting field for study. It is obviously of advantage to the plant that its seeds be disseminated as widely as possible. Sometimes the seeds themselves are modified for dispersal, sometimes the fruit in which they are enclosed and often it is a larger part of the plant. This is the case with the common tumbleweed, a profusely branching plant bearing many seeds which in the fall is torn from its anchorage by the wind and rolled about, scattering its seeds for great distances. Some fruits which are distributed by the wind are provided with wings, as in the maple, elm, ash, etc., while others bear plumes and feathery tufts to enable them to float in the air, as in the dandelions, thistles and others.
Not infrequently the adaptation for seed dispersal is in the mechanical discharge provided for in the structure of the seed-case. In such plants as the witch hazel, violet, wild balsam and others the dry fruits burst with explosive force, throwing the seeds some distance' away from the parent plant.
When the adaptations are related to dispersal by means of animals they take the form of grappling appendages, as in the beggar ticks, stick tights, burdocks, cockleburs, etc., of hard seeds capable of passing through the digestive tract unharmed or of attractive and brightly colored fruits whose seeds are undesirable or indigestible. Good examples of conspicuously colored fruits whose seeds are scattered in this way are cherries, raspberries, blackberries, etc.
It will be noted that the color adaptations of plants referred to so far are supposed to relate plants to animals. But aside from these, there are many general and special adaptations in coloring substances, green, yellow and red, which have a physiological value in plant life. These, however, are special problems of botany and cannot be referred to here.
The whole question of color adaptations in plants has in recent years come up for rediscussion. This discussion cannot be given here, but a good idea of the points at issue can be gathered from the perusal of an essay entitled 'The Significance of Color' by Professor D. T. MacDougal, of the Carnegie Institute.
In discussing the color adaptations of animals, it is convenient to group them in the following classes: Protective and aggressive resemblances; warning coloration; mimicry and colors displayed in courtship.
The color pattern of animals is often such as to effectuall conceal them in their surroundings. Thus tree dwelling animals are often green in color, as is the case with the tree-frog; desert dwellers are often a mottled gray, while arctic dwellers are a snowy white. Far more striking, however, than these cases of general color adaptation are those cases of special adaptation in which the animal resembles in color and shape some particular part of its usual environment. Professors Jordan and Kellogg, in 'Animal Life,' give some good example of special color adaptation. Among them are the following:
The larvae of the geometrid moths, called inch-worms or span worms, are twig like in appearance and have the habit, when disturbed, of standing out stiffly from the twig or branch upon which they rest, so as to resemble in position as well as in color and markings a short or a broken twig. One of the most striking resemblances of this sort is shown by a large geometrid larva found near Ithaca, New York. The body of this caterpillar has a few small, irregular spots or humps, resembling very exactly the scars left by fallen buds or twigs. These caterpillars have a special muscular development to enable them to hold themselves rigidly for long times in this trying attitude. They also lack the middle prop legs of the body common to other lepidopterous larvae, the presence of which would tend to destroy the illusion so successfully carried out by them. The common walking-stick, with its wingless, greatly elongate, dull-colored body, is an excellent example of special protective resemblance. It is quite indistinguishable, when at rest, from the twigs to which it is clinging. Another member of the family of insects to which the walking-stick belongs is the famous green-leaf insect. It is found in South America and is of a bright green color, with broad leaf like wings and body, with markings which imitate the leaf veins and small irregular yellowish spots which mimic decaying, or stained, or fungus covered spots in the leaf.
There are many butterflies that resemble dead leaves. But most remarkable of all is a large butterfly (Kallima) of the East Indian region. The upper sides of the wings are dark, with purplish and orange markings, not at all resembling a dead leaf. But the butterflies when at rest hold their wings together over the back, so that only the under sides of the wings are exposed. The under sides of Kallima's wings are exactly the color of a dead and dried leaf, and the wings are so held that all combine to mimic with extraordinary fidelity a dead leaf still attached to the twig by a short pedicle or leaf-stalk imitated by a short tail on the hind wings and showing mid-rib, oblique veins, and, most remarkable of all, two apparent holes, like those made in leaves by insects, but in the butterfly imitated by two small circular spots free from scales and hence clear and transparent. With the head and feelers concealed beneath the wings, it makes the resemblance wonderfully exact. In all cases of this kind the animals are said to be protectively colored.
Special color resemblance sometimes does more than conceal an animal from its enemies; it often assists it to catch its prey. Such animals are said to be aggressively colored or to have aggressive resemblance. The colors of snakes, lizards and frogs are doubtless both protective and aggressive, while those of the polar bear, the arctic fox, the weasel, the wolf, the lion and the tiger are purely aggressive. Poulton, in 'The Colors of Animals,' cites some examples of a still more remarkable use of color resemblance. He says :
"Special Aggressive Resemblance sometimes does more than hide an animal from its prey; it may even attract the latter by simulating the appearance of some object which is of special interest or value to it. Such appearances have been called Alluring Coloration by Wallace, and they are some of the most interesting of all forms of Aggressive Resemblance.
"An Asiatic lizard (Phrynocephalus mystaceus) is a good example. Its general surface resembles the sand on which it is found, while the fold of skin at each angle of the mouth is of a red color and is produced into a flower-like shape exactly resembling a little red flower which grows in the sand. Insects, attracted by what they believe to be flowers, approach the mouth of the lizard, and are of course captured.
"The Angler, or Fishing Frog, possesses a lure in shape of long, slender filaments, the foremost and longest of which has a flattened and divided extremity. The fish stirs up the mud so as to conceal itself and waves these filaments about. Small fish are attracted by the lure, mistaking it for worms writhing about in the muddy waters ; they approach and are instantly engulfed in the enormous mouth of the Angler.
"An Indian Mantis (Hymenopus bicornis) feeds upon other insects which it attracts by its flower like shape and pink color. The apparent petals are the flattened legs of the insect."
While many animals are very inconspicuously colored, or are manifestly colored so as to resemble their surroundings. generally or specifically, many other animals are very brightly and conspicuously colored and patterned. They possess warning coloration. "A very common example of an animal with warning colors," says Poulton, "is afforded by the larva of the Currant Moth or Magpie Moth, which is excessively abundant in gardens. The caterpillar is extremely conspicuous, being of a cream color, with orange and black markings. Altho it belongs to the group of well-concealed 'stick caterpillars,' it makes no attempt to hold itself in any of the attitudes characteristic of its group. All observers agree that birds, lizards, frogs and spiders either refuse this species altogether or exhibit signs of the most intense disgust after tasting it."
The caterpillar of the Buff-tip Moth and the Cinnabar Moth are also extremely abundant and are good examples of the association of Warning Colors with a nauseous taste. Both of them are gregarious, living in large companies, so that their conspicuous appearance is greatly intensified. The colors of the first named larva are black, yellow and orange. It feeds on oak, elm, lime, birch, hazel, etc., and the large bare branches which attest its appetite are very familiar sights in autumn. The second caterpillar is colored by alternate black and yellow rings; it feeds upon ragwort in the summer. There is plenty of experimental evidence for the unpleasant taste of both caterplilars.
The conspicuously black-and-yellow banded larva of the common Monarch butterfly is a good example of the possession of warning colors by distasteful caterpillars.
These warning colors are possessed not only by the ill tasting caterpillars but by many animals which have special means of defense. The wasps and bees, provided with stings dangerous animals to trouble are almost all conspicuously marked with yellow and black. The lady-bird beetles, composing a whole family of small beetles which are all ill-tasting, are brightly and conspicuously colored and spotted. The Gila Monster, the only poisonous lizard, differs from most other lizards in being strikingly patterned with black and brown. Some of the venomous snakes are conspicuously colored, as the coral snakes or coralillos of the tropics.
All these animals with warning colors are described as possessing some quality, a disagreeable taste or odor, stings, hairs, etc., causing them to be obnoxious to other animals that might seize them for food. Poulton says: "The object of warning colors is to assist the education of enemies, enabling them to easily learn and remember the animals which are to be avoided."
Another special kind of color adaptation to be included under Warning Coloration is known under the head of recognition markings. Instead of attracting the attention of enemies, these markings are of use in attracting the attention of individuals of the same species. To this category belong the white upturned tail of the rabbit, the black tip of the weasel's tail and many of the bright feathers in wings and tail displayed by birds in flight.
Certain animals which are without special means of defense and are not at all formidable or dangerous are yet so marked or shaped and so behave as to present a threatening or 'Terrifying Appearance.' The large green caterpillars of the Sphinx moths the tomato-worm is a familiar one of these larvae have a formidable looking, sharp horn on the back of the next to last body ring. When disturbed they lift the hinder part of the body, bearing the horn, and move it about threateningly. As a matter of fact, the horn is not at all a weapon of defense, but is quite harmless. The larva of the Puss moth has been often referred to as a striking example of terrifying appearances. When one of these larve is disturbed "it retracts its head into the first body ring inflating the margin, which is of bright red color. There are two intensely black spots on this margin in the appropriate position for eyes, and the whole appearance is that of a large flat face extending to the outer edge of the red margin. The effect is an intensely exaggerated caricature of a vertebrate face, which is probably alarming to the vertebrate enemies of the caterpillar.
The effect is greatly strengthened by two pink whips which are swiftly protruded from the prongs of the fork in which the body terminates . . . The end of the body is at the same time curved forward over the back, so that the pink filaments are brandished above the head."
Some of the instances of protective resemblance, warning coloration and "terrifying attitudes" that have been given are sufficiently remarkable, but the phenomena of mimicry are even more surprising. The term mimicry has been reserved for those cases in which an otherwise defenseless animal, one without poison, fang or sting, and without an ill-tasting substance in its body, mimics some other specially defended or inedible animal sufficiently to be taken for it and so escape attack. These instances are especially to be observed among insects. The most familiar example perhaps is that of the Viceroy butterfly, which mimics the Monarch.
The bees and wasps are protected by their stings. They are usually conspicuous, being banded with yellow and black. They are mimicked by numerous other insects, especially moths and flies, two defenseless kinds of in-sects. This mimicking of bees and wasps by flies is very common, and can be observed readily at any flowering shrub. The flower-flies (Syrphidae), which, with the bees, visit flowers, can be distinguished from the bees only by sharp observing. When these bees and flies can be caught and examined in hand it will be found that the flies have but two wings while the bees have four.
In addition to the colors and patterns which assist an animal to evade or warn off its enemies or to seure its prey there are also colors and appendages which must have some very different meaning. These appearances are seen in mature animals, and frequently undergo periodical development at times which correspond to the breeding season; and when the two sexes differ, the males are almost invariably the more brilliant. Instances in which the colors of the males exceed those of the females in brilliancy or pattern are many among fishes, lizards, birds, spiders, insects, etc. It is most common among insects and birds. Many of these sexual differences were described in the chapter on Sexual Selection.
However these colors may have arisen, every observer must admit that they are in some way connected with sex. Darwin accounted for them by his celebrated theory of 'Sexual Selection.' He supposed that the esthetic sense is widely distributed among the higher animals (vertebrates and some of the most specialized invertebrates), and that the colors, which certainly appeal to this sense in man, are not without effect in causing gratification to the animals themselves. This explanation of the origin and meaning of sexual coloring is not accepted by Mr. Wallace, whose chief objection is the lack of evidence that the female has any esthetic preferences at all in the selection of her mate.
Concerning the whole question of the adaptations of plants and animals there is much question today. It is thought by many that the strength of the natural selection explanation rests on the logical nature of its premises and conclusions rather than on scientific observation and experiment. By others it is thought to be the best ex-planation so far advanced. Professor Kellogg comments on the present status of the theory in its relation to adaptations as follows : "There is no gainsaying to the selection explanation its claim to stand among all proposed explanations of adaptation as that one least shaken by the critical attack of it adversaries. However mightily the scientific imagination must exert itself to deliver certain difficult cases into the hands of selection, and however sophisticated and lawyer like the argument from the selection side may be for any single refractory example, the fact remains that the selectionist seems to be able to stretch his explanation to fit all adaptations, with less danger of finding it brought up against positive adverse facts than is possible to the champion. of any other so far proposed explanation."
So comprehensive is the doctrine of evolution that it has been described as "one of the greatest acquisitions of human knowledge." Professor Locy writes of it: "There has been no point of intellectual vantage reached which is more inspiring. It is so comprehensive that it enters into all realms of thought." Weismann expresses the opinion that "the theory of descent is the most progressive step that has been taken in the development of human knowledge," and further he says that this opinion "is justified even by this fact alone: that the evolution idea is not merely a new light on the special regions of biological sciences, zoology and botany, but is of quite general importance. The conception of an evolution of life upon the earth reaches far beyond the bounds of any single science and influences our whole realm of thought. It means nothing less than the elimination of the miraculous from our knowledge of nature, and the placing of the phenomena of life on the same plane as the other natural processes; that is, as having been brought about by the same forces and being subject to the same laws."
Already the field of evolution is being still further ex-tended by new problems in human evolution. The evidences in reference to the evolution of the human body are so conclusive that it is generally accepted that the natural continuity of life includes the human species, and students are turning their attention to the problem of the evolution of mentality. In Professor Locy's words: "The progressive intelligence of animals is shown to depend upon the structure of the brain and the nervous system, and there exists such a finely graded series in this respect that there is strong evidence of the derivation of human faculties from brute faculties."
The conception of evolution is no new idea it is the human idea of history grown larger, large enough to cover the whole world. The extension of the idea was gradual as men felt the need of extending it; and at the same moment we find men believing in the external permanence of one set of phenomena, in the creation of others, in the evolution of others. One authority says human institutions have been evolved; man was created; the heavens are eternal. According to another, matter and motion are eternal; life was created; the rest has been evolved, except, perhaps, the evolution theory which was created by Darwin.
Of the wise men of Greece and what they thought of the nature and origin of things little will be said. Most of them were philosophers, not naturalists, and we are apt to read our own ideas into their words. They thought, indeed; about the physical and organic universe, and some of them believed it to be, as we do, the result of a process; but here in most cases ends the resemblance between their thought and that of modern students.
Thus when Anaximander spoke of a fish like stage in the past history of man, this was no prophecy of the modern idea that a fish like form was one of the far off ancestors of backboned animals; it was only a fancy invented to get over a difficulty connected with the infancy of the first human being.
Or, when several of these sages reduced the world to one element, the ether, it is doing the progress of knowledge an injustice to say that men are simply returning to this after more than two thousand years. For that conception of the ether which is characteristic of modern physical science has been, or is being, slowly attained by precise and patient analysis, whereas the ancient conception was reached by metaphysical speculation. If a return is being made to the Greeks it is on a higher turn of the spiral, so far at least as the ether is concerned.
When Empedocles sought to explain the world as the result of two principles love and hate working on the four elements one may, if so inclined, call these principles "attractive and repulsive forces"; he may recognise in them the altruistic and individualistic factors in organic evolution and what not ; but Empedocles was a poetic philosopher, no far sighted. prophet of evolution.
As in other departments of knowledge, so in biology the work of Aristotle is fundamental. It is wonderful to think of his knowledge of forms and ways of life, or the insight with which he foresaw such useful distinctions as that between analogous and homologous organs, or his recognition of the fact of correlation, of the advantages of division of labor within organims, of the gradual differentiation observed in development. He planted seeds which grew after long sleep into comparative anatomy and classification. Yet with what sublime humility he says : "I found no basis prepared, no models to copy. Mine is the first step, and therfeore a small one, though worked out with much thought and hard labor."
Aristotle was not an evolutionist, for altho he recognised the changefulness of life, the world was to him an eternal fact, not a stage in a process.
"In nature the passage for inanimate things to animals is so gradual that it is impossible to draw a hard-and-fast line between them. After inanimate things come plants, which differ from one another in the degree of life which they possess. Compared with inert bodies, plants seem endowed with life; compared with animals, they seem inanimate. From plants to animals the passage is by no means sudden or abrupt; one finds living things in the sea about which there is doubt whether they be animals or plants."
Among the Romans Lucretius gave noble expression to the philosophy of Epicurus. He was a cosmic, but hardly an organic evolutionist, for according to his poetic fancy organisms arose from the earth's fertile bosom and not by the gradual transformation of simpler predecessors.
"In the first place, the first breed of lions and the savage races their courage has protected, foxes their craft, and stags their proneness to flight. But light-sleeping dogs with faithful heart in breast, and every kind which is born of the seed of beasts of burden, and at the same time the woolly flocks and the horned herds, are all consigned to the protection of man. For they have ever fled with eagerness from wild beasts, and have ensued peace, and plenty of food obtained without their own labor, as we give it in requital of their useful services. But those to whom nature has granted none of these qualities, so that they could neither live by their own means nor per-form for us any useful service, in return for which we should suffer their kind to feed and be safe under our protection, those, you are to know, would lie exposed as a prey and booty of others, hampered all in their own death-bringing shackles, until nature brought that kind to utter destruction.
From Lucretius to Buffon the intervening centuries were uneventful as regards zoology. Hugo Spitzer, one of the historians of evolution, finds analogies between certain medieval scholastics and the Darwinians of the nineteenth century, but these are subtle comparisons. Yet long before Darwin's day there were evolutionists, and the first of these who can be called great was Buffon.
One must guard against supposing that the works of Buffon, or Lamarck, or Darwin were inexplicable creations of genius, or that they came like cataclysms, without warning, to shatter the conventional traditions of their time. For all great workers have their forerunners, who prepare their paths. Therefore in thinking out the history of evolutionist theories before that of Buffon account must be taken of many forces which began to be influential from the twelfth century onward.
Among the evolutionists before Darwin only three need be noted Buffon, Erasmus Darwin and Lamarck. Buffon (1707-1788) was born to wealth and was wedded to For-tune. As Director of the Jardin du Roi he had opportunity to acquire a wide knowledge of animals. He commanded the assistance of able collaborateurs, and his own industry was untiring. He was about forty years old when he began his great Natural History, and he worked till he was fourscore.
Erasmus Darwin (1731-1802), grandfather to the author of the Origin of Species, was a large-hearted, thoughtful physician, whose life was as full of pleasant eccentricities as his stammering speech of wit and his books of wisdom. Buffon underrated the transforming influence of action, and laid emphasis upon the direct influence of surroundings ; Erasmus Darwin emphasized function, and regarded the influence of environment as indirect.
The condition of evolution is variability, or the tendency which animals have to change. The primary factors of evolution are those which produce variations, which cause organic inequilibrium.
A resume of the development of evolution theories should take account of many workers besides Buffon, Erasmus Darwin and Lamarck ; of Treviranus (1776-1837), whose 'Biology' (1802-1805) is full of evolutionary suggestions; of Geoffroy St. Hilaire, who in 1830, before the French Academy of Science, fought with Cuvier an intellectual duel on the question of descent; of Goethe, whose epic of evolution surpasses that of Lucretius; of Oken's speculations ; and of many others in whose works many of the ideas of evolution shone forth.
The status of the theory as it has been worked out by Darwin and his fellow-workers and by modern students has been well worded by Wallace : "Descent with modification is now universally accepted as the order of nature in the organic world. But, while this is true, there remains much uncertainty in regard to the way in which the progressive ascent of life has come about, as to the mechanism of the great nature loom. The relative importance of the various factors in evolution is very uncertain."
There are evidently three direct ways in which these organic changes may be produced: (1) From the nature of the organism itself i.e., from constitutional or germinal peculiarities which are ultimately traceable to influences from without; (2) from changes in its functions or activity in other words, from use and disuse; or (3) from the direct influence of the external conditions of life-food, temperature, moisture, etc.
It is with such problems as these that modern research into the causes of the descent of species is concerned. But of these today scientists are forced to confess almost complete ignorance. As for the future, the keynote of work is to examine life at closer range and from new points of view, to observe life phenomena with a keener and more critical eye, to evolve new methods of work, to experiment more extensively, more intensively and more critically, to classify, to organize and to deduce. And with patient, careful and long-continued effort who will say that even these mysteries may not yield up their secrets for The benefit of mankind?