An excellent example of body degeneration due to the adoption of a parasitic habit is that of _Sacculina_ (fig. 159), a crustacean parasitic on other crustaceans, namely, crabs. The young _Sacculina_ is an active, free-swimming larva essentially like a young prawn or crab. After a short period of independent existence it attaches itself to the abdomen of a crab, and lives as a parasite. It completes its development under the influence of this parasitic life, and when adult bears absolutely no resemblance to such a typical crustacean as a crab or crayfish. Its body external to the host crab is simply a pulsating tumor-like sac, with no mouth-parts, no legs, and internally hardly any well-developed organs except those of reproduction. Degeneration here is carried very far.
[Ill.u.s.tration: FIG. 159.--_Sacculina_, a parasitic crustacean; _A_, attached to a crab, the root-like processes of the parasite penetrating the body of the host; _B_, the active larval condition; _C_, the adult removed from its host. (After Haeckel.)]
Various parasites have been referred to in Part II under their proper branch and cla.s.s. The worms include an unusually large number of them, such as the tapeworms, trichinae and other intestinal forms, all of which live as internal parasites in the alimentary ca.n.a.l or in other organs of higher animals, especially the vertebrates. Many crustaceans are parasitic, usually living, like the fish-lice, as fixed external parasites on fishes, other crustaceans, etc., but with a free and active larval stage. Among the insects, on the contrary, many of the parasitic forms (as the ichneumon flies) are free and active in the adult stage, but live as internal grubs or maggots in the larval stage. The ichneumon flies (of the order Hymenoptera) are four-winged, slender-bodied insects which lay their eggs either on or in (by means of a sharp piercing ovipositor) some caterpillar or beetle grub, into the body of which the young grub-like ichneumon larvae burrow on hatching. The parasites feed on the body-tissues of the host, not attacking, however, such organs as the heart or nervous system, which would produce the immediate death of the host. The caterpillar lives with the ichneumon grubs within it usually until nearly time for its pupation. Often, indeed, it pupates with the parasite still in its body. But it never comes to maturity. The larval ichneumons pupate either within the body of its host, or in a tiny silken coc.o.o.ns outside of its body (fig. 160). From the coc.o.o.ns the winged adult ichneumons issue; and after mating the females find another caterpillar on whose body to lay their eggs.
[Ill.u.s.tration: FIG. 160.--Larva of a sphinx-moth, with coc.o.o.ns of a parasitic ichneumon fly. (From specimen.)]
Degeneration can be produced by other causes than parasitism. It is evident that if for any other reason an animal should adopt an inactive fixed life it would degenerate. The barnacles (see fig. 37) are excellent examples of degeneration through quiescence. They are crustaceans related most nearly to the crabs and shrimps. The young barnacle just from the egg is a six-legged, free-swimming larva (nauplius) with a single eye, greatly like a young prawn or crab. It develops during its independent life two compound eyes and two large antennae. But soon it attaches itself to some stone or sh.e.l.l, or pile or ship"s bottom, giving up its power of locomotion, and its further development is a degeneration. It loses its compound eyes and antennae, and acquires a protecting sh.e.l.l. Its swimming feet become modified into grasping organs, and it loses most of its outward resemblance to the typical members of its cla.s.s. The Tunicata or ascidians compose a whole group of animals which are fixed in their adult condition and have thus become degenerate. They have been likened to a "mere rooted bag with a double neck." In their young stage they are free-swimming, active, tadpole-like or fish-like larvae, possessing organs much like those of the adult simplest fish or fish-like animals. Their larval structure reveals, however, the relationships of the ascidians to the vertebrates, a relationship which is not at all apparent in the degenerate adults. Certain insects live sedentary or fixed lives. All the members of one large family, the Coccidae, or scale-insects (figs.
62 and 63), have females which as adults are wingless and in some cases have no legs, eyes, or antennae, while the males are all winged and have legs and the special sense organs. The males lead a free active life, but the females have nearly or quite given up the power of locomotion, attaching themselves by means of their sucking beak to some plant, where they obtain a sufficient food-supply (plant-sap) and lay their eggs. In both males and females the larvae are little active crawling six-legged creatures with legs, eyes, and antennae.
We are accustomed perhaps to think of degeneration as necessarily implying a disadvantage in life. It is true that a blind, footless, degenerate animal could not cope with the active, keen-sighted, highly organized non-degenerate in free compet.i.tion. But free compet.i.tion is exactly what the degenerate animal has nothing to do with. Certainly the _Sacculina_ and the scale-insects live well; they are admirably adapted to the kind of life they lead. A parasite enjoys certain obvious advantages in life, and even extreme degeneration is no drawback (except as we shall see later), but gives it a body which demands less food and care. As long as the host is successful in eluding its enemies and avoiding accident and injury the parasite is safe. Its life is easy as long as the host lives. But the disadvantages of parasitism and degeneration are nevertheless obvious. The fate of the parasite is bound up with the fate of the host. "When the enemy of the host crab prevails, the _Sacculina_ goes down without a chance to struggle in its own defence. But far more important than the disadvantage in such particular or individual cases is the fact that the parasite cannot adapt itself in any considerable degree to new conditions. It has become so modified, so specialized to adapt itself to the very special conditions under which it now lives, it has gone so far in giving up organs and functions, that if present conditions change and new ones come to exist the parasite cannot adapt itself to them. The independent free-living animal holds itself, one may say, able and ready to adapt itself to any new conditions of life. The parasite has risked everything for the sake of a sure and easy life under the present existing conditions. Change of conditions means its extinction."
[Ill.u.s.tration: FIG. 161.--Young fur seals, _Callorhinus ursinus_, of the Tolstoi rookery, St. Paul Island, Bering Sea, killed by a parasitic intestinal worm, _Uncinaria_ sp. (Photograph by the Fur Seal Commission.)]
For an elementary account of commensalism and parasitism see Jordan and Kellogg"s "Animal Life," pp. 172-200. The account here given is based on the author"s previously written account in "Animal Life." See also Van Beneden"s "Animal Parasites and Messmates."
CHAPTER x.x.xI
COLOR AND PROTECTIVE RESEMBLANCES
TECHNICAL NOTE.--For an appreciation of the reality of protective resemblances observations must be made in the field. Examples are easily found. Locusts, katydids, green caterpillars, lizards, crouching rabbits, and brooding birds are readily observed instances of general protective resemblance. For examples of variable resemblance examine specimens of a single locust species taken from different localities; the individuals of the various species of the genus _Trimerotropis_ show much variation to harmonize with their surroundings. Collect a number of larvae (caterpillars) of one of the swallow-tail b.u.t.terflies (_Papilio_), and when ready to pupate put them separately into pasteboard boxes lined inside with differently colored paper. The chrysalids will show in their coloration the influence of the different colors of the lining paper, their immediate environment. As examples of special protective resemblance observe inch- or span-worms (larvae of Geometrid moths). The walking-stick is not uncommon; many spiders that inhabit flower-cups show striking protective color patterns; and the Graptas or comma-b.u.t.terflies which resemble dead leaves may be examined.
To ill.u.s.trate warning colors, find, if possible, the larvae (caterpillars) of the common milkweed or monarch b.u.t.terfly (_Anosia plexippus_), and offer them to birds, at the same time offering other caterpillars, and note the results. For terrifying or threatening appearance find specimens of the large green tobacco- or tomato-worm (larva of the five-spotted sphinx-moth, _Phlegethontius carolina_), or other sphingid larvae.
The b.u.t.terflies ill.u.s.trating the striking example of mimicry, described on p. 432, can be found in most parts of the country.
Syrphid and other flies which mimic bees and wasps can readily be found on flowers.
Each student should search for himself for examples of protective resemblance.
=Use of color.=--The prevalence of color and the oftentimes striking and intricate coloration patterns of animals demand some explanation.
As naturalists are accustomed to find the frequently bizarre and seemingly inexplicable shapes and general structure of animals readily explained by the principle of adaptation, that is, special modification of body-structure to fit special conditions of life, so they look to use as the chief explanation of color and markings. Some uses are obvious; bright colors and striking patterns may serve to attract mates or to avail as recognition marks by which individuals of a kind may readily recognize each other. The white color of arctic animals probably serves to help keep them warm by preventing radiation of heat from the body; on the other hand dark color may also help to keep animals warm by absorbing heat. "But by far the most widespread use of color is for another purpose, that of a.s.sisting the animal in escaping from its enemies or in capturing its prey."
It is common knowledge that the young and old, too, of many kinds of ground-inhabiting animals, when startled by an enemy will not run, but crouching close to the ground remain immovable, trusting to remain unperceived. But a blue or crimson rabbit, however still it might keep, would be easily seen by its enemy and killed. Rabbits, however, which are good examples of animals having this habit of lying close, are neither blue nor green nor red, but are colored very much like the ground on which they crouch. This harmonious coloration is as necessary to the success of this habit as is the keeping still. A gra.s.shopper flying or leaping in the air is conspicuous; when it alights how inconspicuous it is! Unless one has followed it closely in its flight and has kept the eye fixed on it after alighting it is usually impossible to distinguish it from its surroundings. And this is greatly to the advantage of the gra.s.shopper in its efforts to escape its enemies, that is, in its struggle for existence. On the other hand a green katydid would be very conspicuous in a dusty road.
But dusty roads are precisely where katydids do not rest. They alight among the green leaves of a tree or shrub. The animals that live in deserts are almost all obscurely mottled with gray and brownish and sand-color so as to harmonize in color with their habitual environment. The arctic hares and foxes and grouse which live in regions of perpetual snow are pure white instead of red or brown or gray like their cousins of temperate and warm regions.
These cases of an animal"s color and markings harmonizing with the usual environment are called instances of _protective resemblance_; that is, they are resemblances for a purpose, that purpose being to render the animal indistinguishable from its surroundings and thus to aid it in escaping its enemies. Such protective resemblances are obviously of great value to animals, and, like other advantageous modifications, have been produced by the action of natural selection.
Those individuals of a species most conspicuous and hence most readily perceived by enemies are the first (under ordinary circ.u.mstances) to be captured and eaten. The less conspicuous live and produce young like themselves. Of these young the least conspicuous are again saved and so over and over again through thousands of generations until this natural selecting of the protectively colored results in the production of the wonderfully specialized examples of resemblance to which attention is called in the following paragraphs.
=General, variable, and special protective resemblance.=--In the brooks most fishes are dark olive or greenish above and white below.
To the birds and other enemies which look down on them they are colored like the bottom. To their fish-enemies which look up from below they are like the white light above them in color and their forms are not clearly seen. The green tree-frogs and tree-snakes which live habitually among green foliage; the mottled gray and tawny lizards and birds and small mammals of the plains and deserts, and the white hares and foxes and owls and ptarmigan of the snowy arctic regions--all show a general protective resemblance.
[Ill.u.s.tration: FIG. 162.--The twig or walking-stick insect, _Diapheromera femorata_. (From specimen.)]
Sometimes an animal changes color when its surroundings change.
Certain hares and grouse of northern lat.i.tudes are white in winter when the snow covers all the ground, but in summer when much of the snow melts, revealing the brown and gray rocks and withered leaves, they put on a grayish and brownish coat of hair or feathers. A small insect called the toad-bug (_Galgulus_) lives abundantly on the banks of a pond on the campus of Stanford University. The sh.o.r.es of this pond are covered in some places with bits of bluish rock, in others with bits of reddish rock, and in still others with sand. Specimens of the toad-bug collected from the blue rocks are bluish or leaden in color, those from the red rocks are reddish, and those from the sand are sand-colored. Changes of color to suit the surroundings can be quickly made by some animals. The chameleons of the tropics change momentarily from green to brown, blackish, or golden. There is a little fish (_Oligocottus snyderi_) common in the tide-pools of the Bay of Monterey in California whose color changes quickly to harmonize with the rocks it happens to rest above. Such changing coloration to suit the surroundings may be called _variable protective resemblance_.
Very striking are those cases of protective resemblance in which the animal resembles in color and shape, sometimes in extraordinary detail, some particular object or part of its usual environment. This may be called _special protective resemblance_. 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 on which they rest, so as to resemble in att.i.tude as well as color and markings a short or broken twig. To increase this simulation the body of the larva often has a few irregular spots or humps resembling the scars left by fallen leaves, and it also lacks the middle prop-legs of the body common to other lepidopterous larvae, which would tend to destroy the illusion so successfully carried out by it. The common twig-insect or walking-stick (fig. 162) with its wingless, greatly elongate, brown or greenish body and legs is when at rest quite indistinguishable from the twigs on which it lies. Another excellent example of special protective resemblance is furnished by the famous green-leaf insect (_Phyllium_) of the tropics, which has broad leaf-like wings and body of a bright green color with markings which imitate the leaf-veins, and small irregular yellowish spots which simulate decaying or stained or fungus-covered spots in the leaf. Most striking of all, however, is the large dead-leaf b.u.t.terfly _Kallima_ (fig. 163) of the East Indian region. The upper sides of the wing are dark with purplish and orange markings not at all resembling a dead leaf. But the b.u.t.terflies when at rest hold their wings together over the back, so that only the under sides of them are exposed. These are exactly the color of a dry dead leaf with markings mimicking midrib and oblique veins, and, most remarkable of all, what are apparently two holes like those made in leaves by insects, but in the b.u.t.terfly imitated by two small circular spots free from scales and hence clear and transparent. When _Kallima_ alights it holds the wings in such position that the combination of all four produces with remarkable fidelity the simulation of a dead leaf still attached to the twig by a short pedicel or leaf-stalk (imitated by a short "tail" on the hind wings). The head and legs of the b.u.t.terfly are concealed beneath the wings.
[Ill.u.s.tration: FIG. 163.--The dead-leaf b.u.t.terfly, _Kallima_ sp., a remarkable case of special protective resemblance. (From specimen.)]
=Warning colors, terrifying appearances, and mimicry.=--While many animals are so colored as to harmonize with their habitual or usual environment, others on the contrary are very brightly colored and marked in such bizarre and striking pattern as to be conspicuous. There is no attempt at concealment; it is obvious that conspicuousness is the object sought or at least produced by the coloration. Animals like these, we shall find, are in almost all cases specially protected by special weapons of defence such as stings or poison-fangs, or by the secretion of an acrid, ill-tasting fluid in the body. Many caterpillars have been found, by observation in nature and by experiment, to be distasteful to insectivorous birds. Now it is obvious that it would be advantageous to these caterpillars to be readily recognized by birds. After a few trials the bird learns by experience to let these distasteful larvae alone; their conspicuous markings serve as _warning colors_. The black-and-yellow-banded caterpillar of the common milkweed or monarch b.u.t.terfly (_Anosia plexippus_) is a good example of such protection by a combination of distastefulness and warning coloration. The little lady-bird beetles are mostly distasteful to birds; they are brightly and conspicuously marked. Certain little Nicaraguan frogs have a bright livery of red and blue, in strong contrast to the dull concealing colors of other frogs in their region. By offering these little blue and red frogs to hens and ducks the naturalist Belt found that they are distasteful to the birds.
[Ill.u.s.tration: FIG. 164.--The larva of the pen-marked sphinx-moth, _Sphinx chersis_, showing terrifying att.i.tude. (After Comstock.)]
Certain animals which are without special means of defence and are not distasteful 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- and tobacco-worms, are familiar examples, each larva having a sharp horn on the back of the next to last body-segment (fig. 164). When disturbed the caterpillar a.s.sumes a threatening att.i.tude, and the horn seems to be an effective weapon of defence. As a matter of fact it is not at all a weapon of defence, being weak, not provided with poison, and altogether harmless.
But it would plainly be to the advantage of a defenceless animal, one without poison-fangs or sting and without an ill-tasting substance in its body, to be so marked and shaped as to mimic some other specially defended or inedible animal sufficiently to be mistaken for it and thus to escape attack. Such cases have been noted, especially among insects. This kind of protective resemblance may be called _mimicry_.
A most striking case is that presented by the familiar monarch and viceroy b.u.t.terflies (fig. 165). The monarch (_Anosia plexippus_) is perhaps the most abundant and widespread b.u.t.terfly of our country. It is a fact well known to entomologists that it is distasteful to birds and is let alone by them. It is conspicuous, being large and chiefly red-brown in color. The viceroy (_Basilarchia archippus_), also red-brown and patterned almost exactly like the monarch, is not, as its appearance would seem to indicate, a very near relation of the latter, but on the contrary it belongs to a genus of b.u.t.terflies all of which, except the viceroy and one other, are black and white in color and of different pattern from the monarch. The viceroy is not distasteful to birds, but by its extraordinary simulation or mimicking of the monarch it is not distinguished from it and so is not molested.
In the tropics there have been discovered numerous examples of mimicry among insects. The members of two large families of b.u.t.terflies (Danaidae and Heliconidae) are distasteful to birds and are mimicked by members of other b.u.t.terfly families (especially the Pieridae).
[Ill.u.s.tration: FIG. 165.--The monarch b.u.t.terfly, _Anosia plexippus_ (above), distasteful to birds, and the viceroy, _Basilarchia archippus_ (below), which mimics it. (From specimens.)]
=Alluring coloration.=--A few animals show what is called alluring coloration; that is, they display a color pattern so arranged as to resemble or mimic a flower or other lure, and thus entice to them other animals, their natural prey. Certain Brazilian fly-catching birds have a brilliantly colored crest which can be displayed in the shape of a flower-cup. The insects attracted by the false flower furnish the bird with food. In the tribe of fishes called the "anglers" or "fishing frogs," the front rays of the dorsal fin are prolonged in the shape of long slender filaments, the foremost and longest of which has a flattened and divided extremity. The angler conceals itself in the mud or in the cavities of a coral reef, and waves the filament back and forth. Small fish are attracted by the lure, mistaking it for worms writhing about. When they approach they are engulfed in the mouth of the angler, which in some species is of enormous size. One of these angler species is known to fishermen as the "all-mouth."
For a fuller account of protective resemblances and mimicry see Jordan and Kellogg"s "Animal Life," pp. 201-223. For still more extended accounts see Poulton"s "Colours of Animals," and Beddard"s "Animal Coloration."
CHAPTER x.x.xII
THE DISTRIBUTION OF ANIMALS
TECHNICAL NOTE.--The larger aspects or phenomena of the distribution of animals over the earth on land and in sea cannot be studied personally in the field by the student, but many local features of distribution can be so observed and studied. The restriction of certain kinds of animals to certain kinds of habitat, the presence and character and effectiveness of barriers, some of the modes of distribution, the presence and successful life of introduced foreign species such as the black and brown rats, the English sparrow, the German and Asiatic c.o.c.kroaches, the gradual change of range or distribution of certain kinds of animals through the influence of a change in environment (caused by man in cutting off forests, cultivating heretofore wild pastures, etc.) all offer favorable and profitable opportunities for personal observation.
An excellent and feasible piece of field-work in distribution is the making of a zoological survey of the locality in which the school is situated. A map of the locality should be made on a generous scale, which should include all prominent physical features of the region, such as streams, ponds, hills, woodlands, marshes, etc., and on this map should be indicated the places where the various animals of the local fauna occur. Some of the animal species will have a limited range, and the limits of this range should be shown. This map and faunal list can be added to and perfected by successive cla.s.ses. For fuller discussions of the geographical distribution of animals see Jordan and Kellogg"s "Animal Life," Beddard"s "Zoogeography," Heilprin"s "The Distribution of Animals," and Wallace"s "Geographical Distribution."
=Geographical distribution.=--It is a matter of common knowledge with all of us that there are no wild lions or camels or kangaroos or monkeys or ostriches or nightingales in North America. Ostriches are found only in Africa and South America, kangaroos only in Australia, lions only in Asia and Africa. On the other hand there are no opossums in Europe or grizzly bears or rattlesnakes anywhere else in the world than in this country. That is, certain kinds of animals have a certain limited range of occurrence or distribution. It is obvious, too, that certain animals live only on land, while others live only in water, and of these latter some are restricted to the ocean, while others live only in fresh water.
All of the facts regarding the dispersion or diffusion of animals on land and in water make up the science of the _geographical distribution of animals_, or, as it is sometimes called, _zoogeography_. Under this subject are included not only the facts of the present actual distribution or occurrence of animals over the world, but the facts concerning the reasons for this distribution, the modes of travel and dispersion, the character and influence of barriers to the spread, and the results, in the adaptation of old forms and the production of new forms, of the phenomena of distribution.
Just as maps are made to show graphically the facts of political geography, which concerns the position and extent of the various powers and States which claim the allegiance of the people, and the facts of physical geography, which concerns the physical character of the earth"s surface, so maps are made to show the geographical distribution of animals. Because of the great numbers of animal species no one map can show the distribution of all species, but a series of maps of the world or of a continent or of a State or county or more limited region could be made (and many such have been made) showing the distribution of selected species. In a map of a limited locality, say of a few square miles, the occurrence and distribution of most of the commoner and more familiar animals can be shown, and each high school should possess such a map (see technical note at beginning of this chapter).
=Laws of distribution.=--The laws governing the distribution of animals over the earth"s surface have been recently[20] expressed in a simple statement as follows: Every species of animal is found in every part of the earth unless (_a_) its individuals have been unable to reach this region on account of barriers of some sort; or (_b_) having reached it, the species is unable to maintain itself, through lack of capacity for adaptation, through severity of compet.i.tion with other forms, or through destructive conditions of environment; or (_c_) having entered and maintained itself it has become so altered in the process of adaptation as to become a species distinct from the original type.
=Modes of migration and distribution.=--That animals should be continually trying to extend their range is obvious from what we know of their rapid increase by multiplication. In a region which can provide food for but one thousand wolves, there is a production each year of several times one thousand. These new wolves must struggle among themselves for food, or migrate, if possible, to new regions as yet not inhabited by wolves. The wolf"s mode of migration or distribution is walking or running, and so with other mammals except the bats and aquatic forms. Birds and bats can fly, and can thus migrate more swiftly, farther, and over barriers which would stop mammals. Most insects can fly. Worms can only crawl and very slowly at that. Fishes can swim, but if they are in a landlocked sheet of water, they cannot go beyond its confines. Marine animals can migrate from ocean to ocean, and land animals from continent to continent unless checked by barriers (see next paragraph).
But besides such voluntary and independent modes of distribution long journeyings may be made involuntarily, or by a pa.s.sive migration as it may be called. Parasites, for example, are carried by their hosts in all their travels; the tiny Tardigrada and Rotifera, which can be desiccated and yet restored to active life by coming again into water, are carried in the dried mud on the feet of birds or other animals. On floating objects in rivers or in ocean currents land-animals may be carried long distances. Man, the greatest traveller of all, is responsible for the widened distribution of many animals. Thus have come to us in ships from Europe the black and brown rats, the English sparrow, the Hessian fly, the commonest c.o.c.kroaches of our houses and many other forms. And these animals have been carried involuntarily all over the United States in railway-cars and wagons.
=Barriers to distribution.=--As is indicated in the paragraph on the modes of migration, a considerable sheet of water is obviously a barrier to the further travelling of a walking or crawling land-animal, although no barrier to a winged form. Similarly a strip of land is a barrier to a strictly aquatic animal as a fish. Or a high fall in the stream may serve as an insuperable barrier, making it impossible for any fish below the fall to reach the upper part of the stream. Numerous cases of this kind are known in the Rocky Mountains and Sierra Nevada, where a stream may be well supplied with trout below a fall, and quite bare of these fish above the fall. In the Yosemite Valley in California trout live in the Merced River below the great Vernal and Nevada falls, but above these falls the Merced contains no trout. To fresh-water swimming animals salt water may be a barrier; thus some kinds of fresh-water fishes are limited to one of two near-by streams although the mouths of these streams empty near each other into the ocean. The amphibious batrachians, at home in fresh water and on land, are killed by contact with sea-water.
Earthworms also are killed by salt water. Thus the narrowest ocean strait is as effective a barrier to these animals as a whole sea.
High mountain ranges and broad deserts are barriers to many land-animals, partly because of the physical obstacles, partly because of the differences in temperature and in food-supply.
Temperature and climate (as distinct from temperature) and the ocean are the three great barriers when we consider the animal kingdom as a whole, and look for the causes which determine the chief zoogeographical divisions of the earth"s surface. Most of the tropical animals cannot endure frost, hence the isothermal line of frost is a line across which few tropical animals venture. Most arctic animals are enfeebled by heat, and the isothermal line which marks off the region in which frost occurs the year round is another great zoogeographical boundary. But while these lines are limits for localized species, some animals, as birds, especially, keep within a relatively uniform temperature by migrations across these lines. It should be borne in mind that the gradual decrease in temperature met with in going north or south from the tropics is also met in the ascent of high mountains. The summits of lofty peaks, even in the tropics, are truly arctic in character; they are snow-covered, and the animals and plants on them are truly arctic. Thus in the ascent of a single mountain a whole series of life-zones from tropical to arctic can be traversed.