Elementary Zoology Part 25

The lowest Primates are the lemurs found in Madagascar, in which island they include about one-half of all the mammalian species found there. The brain is much less developed in the lemurs than in any of the other monkeys. The monkeys and apes may be divided into two groups, the lower, platyrrhine monkeys, found in the New World, and the higher, catarrhine forms, limited to the Old World. The platyrrhine monkeys have wide noses in which the nostrils are separated by a broad septum and with the openings directed laterally.

These monkeys are mostly smaller and weaker than the Old World forms and are always long-tailed, the tail being frequently prehensile. They include the howling, squirrel, spider, and capuchin monkeys common in the forests of tropical South America. The catarrhine monkeys have the nose-septum narrow and the openings of the nostrils directed forwards, and the tail is wanting in numerous members of the group. They include the baboons, gorillas, orang-outangs, and chimpanzees. These apes have a dent.i.tion approaching that of man, and in all ways are the animals which most nearly resemble man in physical character.

PART III

ANIMAL ECOLOGY

CHAPTER XXIX

THE STRUGGLE FOR EXISTENCE, ADAPTATION, AND SPECIES-FORMING

TECHNICAL NOTE.--Multiplication, or increase by geometric ratio, among animals can be ill.u.s.trated by noting the many eggs laid by a single female moth or beetle or fly or mosquito or any other common insect (or almost any other non-mammalian animal). The production of many live young by each female rose aphid can be readily seen; the number of young in a litter of kittens or pups or rabbits is a good ill.u.s.tration. From this geometric increase it is obvious that there must be a great crowding of animals and a struggle among them for existence. This struggle and the downfall of the many and success of the victorious few can be observed by rearing in a small jar of water all the young of a single brood of water-tigers (larva of _Dyticus_) or other aquatic predaceous insect. The strongest young will live by killing and eating the weaker of their own kind. In a spider's egg-sac the young after hatching do not immediately leave the sac, but remain in it for several days. During this time they live on each other, the strongest feeding on the weaker. Thus out of many spiderlings hatched in each sac comparatively few issue. This can be readily observed. Open several egg-sacs and count the eggs in them. Let the spiderlings hatch and issue from some other egg-sacs belonging to the same species of spider. The number of issuing spiderlings will always be much less than that of the eggs. The actual working of natural selection and the forming of new species can of course be seen only in results, and not in process. The great variety of adaptation, the fitness of adaptive structures, can be readily ill.u.s.trated among the commonest animals. Animals showing certain striking and unusual adaptations will perhaps make the matter more obvious. To all teachers will occur numerous opportunities of ill.u.s.trating, by reference to actual processes or to obvious results, the principles of this chapter.

=The multiplication and crowding of animals.=--In the reproduction or multiplication of animals the production of young proceeds in geometric ratio, that is, it is truly a multiplication. Any species of animal, if its multiplication proceeded unchecked, would sooner or later be sufficiently numerous to populate exclusively the whole world. The elephant is reckoned the slowest breeder of all known animals. It begins breeding when thirty years old and goes on breeding until ninety years old, bringing forth six young in the interval, and surviving until a hundred years old. Thus after about eight hundred years there would be, if all the individuals lived to their normal age limit, 19,000,000 elephants alive descended from the first pair. A few years more of unchecked multiplication of the elephant and every foot of land on the earth would be covered by them. But the rate of multiplication of other animals varies from a little to very much greater than that of the elephant. It has been shown that at the normal rate in increase in English sparrows, if none were to die save of old age, it would take but twenty years to give one sparrow to every square inch in the State of Indiana. The rate of increase of an animal, each pair producing ten pairs annually and each animal living ten years, is shown in the following table:

Years. Pairs produced. Pairs alive at end of year.

1 10 11 2 110 121 3 1,210 1,331 4 13,310 14,641 5 146,410 161,051 10 ...... 25,937,424,600 20 ...... 700,000,000,000,000,000,000

Some animals produce vast numbers of eggs or young; for example, the herring, 20,000; a certain eel, several millions; and the oyster from 500,000 to 16,000,000. Supposing we start with one oyster and let it produce one million of eggs. Let each egg produce an oyster which in turn produces[19] one million of eggs, and let these go on increasing at the same rate. In the second generation there would be one million million of oysters, and in the fourth, i.e. the great great grandchildren of the first oyster, there would be one million million million million of oysters. The sh.e.l.ls of these oysters would just about make a ma.s.s the size of the earth.

But it is obvious that all the new individuals of any animal produced do not live their normal duration of life. All animals produce far more young than can survive. As a matter of fact, which we may verify by observation, the number of individuals of animals in a state of nature is, in general, about stationary. There are about as many squirrels in the forest one year as another, about as many b.u.t.terflies in the field, about as many frogs in the pond. Some species increase in numbers, as for example, the rabbit in Australia, which was introduced there in 1860 and in fifteen years had become so abundant as to be a great pest. Other species decrease, as the buffaloes, which once roamed our great plains in enormous herds and are now represented by a total of a few hundred individuals, and the pa.s.senger-pigeon, whose migrating flocks ten years ago darkened the air for hours in parts of the Mississippi valley, where now it is a rare bird. But the hand of man is the agent which has helped to increase or to check the multiplication of these animals. In nature such quick changes rarely occur.

=The struggle for existence.=--The numbers of animals are stationary because of the tremendous mortality occasioned by the constant preying on eggs and young and adults by other animals, because of strenuous and destructive climatic and meteorological conditions, and because there is not s.p.a.ce and food for all born, not even, indeed, for all of a single species, let alone all of the hundreds of thousands of species which now inhabit the earth. There is thus constantly going on among animals a fearful _struggle for existence_. In the case of any individual this struggle is threefold: (1) with the other individuals of his own species for food and s.p.a.ce; (2) with the individuals of other species, which prey on him, or serve as his prey, or for food and s.p.a.ce; and (3) finally with the conditions of life, as with the cold of winter, the heat of summer, or drouth and flood. Sometimes one of these struggles is the severer, sometimes another. With the communal animals the struggle among individuals is lessened--they help each other; but when the struggle with the conditions of life are easiest, as in the tropics or in the ocean, the struggle among individuals becomes intensified. Each strives to feed itself, to save its own life, to produce and safeguard its young. But in spite of all their efforts only a few individuals out of the hosts produced live to maturity. The great majority are destroyed in the egg or in adolescence.

=Variation and natural selection.=--What individuals survive of the many which are born? Those best fitted for life; those which are a little stronger, a little swifter, a little hardier, a little less readily perceived by their enemies, than the others. They are the winners in the struggle for existence; they are the survivors. And this survival of the fittest, as it is called, is practically a process of selection by Nature. Nature selects the fittest to live and to perpetuate the species. Their progeny again undergo the struggle and the selecting process, and again the fittest live. And so on until adjustment or harmonizing of animals' bodies and habits with the conditions of life, with their environment, comes to be extremely fine and nearly perfect.

It is evident, of course, that such a natural selection or survival of the fittest and consequent adaptation to environment presupposes differences among the individuals of a species. And this is an observed fact. No two individuals, although of the same brood, are exactly alike at birth; there always exist slight variations in structure and performance of functions. And these slight variations are the differences which determine the fate of the individual. One individual is a little larger or stronger or swifter or hardier than its mates. The existence of this variation we know from our observation of the young kittens or puppies of a brood. So it is with all animals. Thus natural selection depends upon two factors, namely, the excess in the production of new individuals and the consequent struggle for existence among them, and the existence of variations which give certain individuals slight advantages in this struggle.

=Adaptation and adjustment to surroundings.=--The action of natural selection obviously must, and does, result in a fine adaptation and adjustment of the structure and habits of animals to their surroundings. If a certain species or group of individuals cannot adapt itself to its environment, it will be crowded out by others that can. A slight advantageous variation comes in time by the continuously selective process to be a well-developed adaptation.

The diverse forms and habits possessed by animals are chiefly adaptations to their special conditions of life. The talons and beak of the eagle, the fishing-pouch of the pelican, the piercing chisel-like bill of the woodp.e.c.k.e.r, and the sensitive probing-bill of the snipe are adaptations connected with the special feeding habits of these birds. The quills of the porcupine, the poison-fangs of the rattlesnake, the sting of the yellow-jacket, and the antlers of the deer are adaptations for self-defence. The fins and gills of fishes, the shovel-like fore feet of the mole, the wings of birds and insects and bats, the toe-pads of the tree-toad, the leaping-legs of the gra.s.shopper, all these are adaptations concerned with the special life-surroundings of these animals.

Adaptations may relate to habits and behavior as well as to structure.

Plainly adaptive are such habits as the migration of birds and some other animals, most of the habits connected with food-getting, and especially striking and interesting those connected with the production and care of the young, including nest-making and home-building.

=Species-forming.=--It is evident that through the c.u.mulative action of natural selection, animals of a structural type considerably (even unlimitedly) different from any original type may in time be produced by the gradual modification of the original type under new conditions.

If, for example, a few individuals of a mainland species should come to be thrown as waifs of wave and storm upon an island, and if these should be able to maintain themselves there and produce young, increasing so as to occupy the new territory, there would be produced in time a new type of individual conforming or adapted to the conditions obtaining in the island, these conditions being, of course, almost certainly different from those of the mainland. Thus as an offshoot or derivation from the original type still existing on the mainland we should have the new island-inhabiting type. Now when these island individuals come to differ so much, structurally and physiologically, from the mainland type that they cannot, even if opportunity offers, successfully mate or interbreed with mainland individuals the island type const.i.tutes a new species. That is, our distinction between species rests not only on structural differences, but on the impossibility of interbreeding (at least for the production of fertile young). Such a combination of the action of natural selection and the condition of isolation (as ill.u.s.trated by the case of island animals), is probably the most potent factor in the production of new species of animals (and plants).

For accounts of the struggle for existence, variations, adaptations, natural selection and species-forming see Darwin's "Origin of Species,"

Wallace's "Island Life," and Romanes' "Darwin and After Darwin," I.

=Artificial selection.=--When a selection among the individuals of a species, that is, the choosing and preserving of individuals which show a certain trait or traits and the destroying of those individuals not possessing this trait, is done by man, it is called artificial selection. To artificial selection we chiefly owe all the many races or varieties of our domesticated animals and plants. For example, from the ancestral jungle fowl have been developed by artificial selection (and by cross-breeding) all our kinds of domestic fowl, as Brahmas, black Spanish, bantams, game-c.o.c.ks, etc.; from the wild rock-dove have been developed our various fancy pigeons, as carriers, pouters, fantails, etc.

For an account of artificial selection see Darwin's "Plants and Animals under Domestication," and Romanes' "Darwin and After Darwin," I.

FOOTNOTE:

[19] Oysters are hermaphroditic, each individual producing both sperm- and egg-cells.

CHAPTER x.x.x

SOCIAL AND COMMUNAL LIFE, COMMENSALISM AND PARASITISM

=Social life and gregariousness.=--TECHNICAL NOTE.--Students should refer to examples of gregariousness from their own observations of animals. The roosting together of crows and of blackbirds; the gathering of swallows preparatory to migration; the flocking of geese and ducks, with leaders, in their migratory flights, all can be readily observed. From observation or general reading students will be more or less familiar with prairie-dog villages, beaver-dams and marshes, the one-time great herds of bison, etc.

The struggle for existence is always operative; but in some cases one or more phases of it may be ameliorated. For example, the amelioration of the struggle among individuals of one species obtains in a lesser or greater degree in the case of those animals which exhibit a _social life_, of which mutual aid and mutual dependence are the basis. The honey-bee and the ants are familiar examples of animals which show a high degree of social life. They live, indeed, a truly communal life, where the fate of the individual is bound up in the fate of the community. But there are many animals which show a much lower degree of mutual aid and a far less coherent society. The simplest form of social life exists among those animals in which many individuals of one species keep together, forming a great band or herd. In this case there is not nearly so much mutual aid or mutual dependence as in that of the honey-bee, and the safety of the individual is not wholly bound up in the fate of the herd. Such animals are said to be _gregarious_ in habit, and this gregariousness is undoubtedly advantageous to the individuals of the band. The great herds of reindeer in the North, and of the bison or buffalo which once ranged over the Western American plains are examples of a gregariousness in which mutual protection from enemies, like wolves, seems to be the princ.i.p.al advantage gained.

The bands of wolves which hunted the buffalo show the advantage of mutual help in aggression as well as in protection. Prairie-dogs live in great villages or communities which spread over many acres. By shrill cries they tell each other of the approach of enemies, and they seem to visit each other and to enjoy each other's society a great deal, although that they are thus afforded much actual active help is not apparent. The beavers furnish a well-known and very interesting example of mutual help; they exhibit a communal life although a simple one. They live in villages or communities, all helping to build the dam across the stream which is necessary to form the marsh or pool in which the nests or houses are built.

=Communal life.=--TECHNICAL NOTE.--See technical notes, pp. 212 _et seq_, for directions for work in connection with the study of the communal life of ants, bees, and wasps.

When many individuals of a species live together in a community in which the different kinds of work are divided more or less distinctly among the different members and where each individual works primarily for the whole and not for himself; where there is, in other words, a thorough mutual help and mutual dependence among the members of the community accompanied by a division of labor, the life of the species is truly communal. Those animals which show the most elaborate and specialized _communal life_ are the termites or white ants, the social bees and wasps, and the true ants. Of these the ants and honey-bees stand first.

As already explained (see pp. 220 _et seq_), there are among these communal insects several different kinds of individuals in each species.

With most animals there are two kinds only, males and females, which may or may not show differences in color, form, etc., so that they are readily distinguishable. Among all the communal insects, however, there are always three kinds of individuals, males, females, and workers, these last being infertile individuals. With the social wasps and social bees the workers are all infertile females and are smaller than the fertile forms; with the termites there are besides the fertile males and females, which are winged, workers which are wingless, and also peculiar wingless individuals called soldiers which have very large jaws and whose business it is to fight off attacking enemies of the community.

Among the ants the workers are also wingless, while the males and females are winged. The worker ants in many species are of two kinds, so-called worker majors and worker minors, differing markedly in size.

All the ant workers are good soldiers, but with some the fighting is done almost wholly by certain especially large-headed and large-jawed ones which may be called soldier-workers.

Thus among all strictly communal animals there is a specialization or differentiation of individuals accompanying the division of labor.

Special individuals have a certain part of the work of the community to do, and they are specially modified in structure to do this work.

This structural modification may make them incapable of performing certain other labor or work which is necessary for their living and which must be done for them, therefore, by others. Thus the mutual interdependence of the individuals composing a colony is very real.

The worker honey-bees cannot perpetuate the species; honey-bees would die out were it not for the males and females. But the males and females have given up the functions of food-getting and of caring for their young; did not the workers do these things for them, the community would die out quite as soon.

The advantages of communal or social life, of co-operation and mutual aid are real. Those animals that have adopted such a life are among the most successful of all in the struggle for existence. The termite worker is one of the most defenseless and for those animals that prey on insects one of the most toothsome insects, and yet the termite is one of the most abundant and successfully living insect kinds in all the tropics. Ants are everywhere and are everywhere successful. The honey-bee is a popular type of successful life. The artificial protection afforded it by man may aid it in its struggle for existence, but it gains this protection because of certain features of its communal life, and in nature the honey-bee takes care of itself well. Co-operation and mutual aid are among the most important factors which help in the struggle for existence.

=Commensalism.=--TECHNICAL NOTE.--Examine ants' nests to find myrmecophilous insects. If on the seash.o.r.e search for hermit-crabs with sea-anemones on sh.e.l.l. If inland, try to have some preserved specimens showing the crabs and sea-anemones.

The phases of living together and mutual help just discussed concerned in each instance a single species of animal. All the members of a pack of wolves or of a honey-bee community belong to a single species. But there are numerous instances known of the mutually advantageous a.s.sociation of individuals of two different species. Such an a.s.sociation is called _commensalism_ or _symbiosis_.

The hermit-crabs live, as has been learned (p. 154), in the sh.e.l.ls of molluscs, most of the body of the crab being concealed within the sh.e.l.l, only the head and the grasping and walking legs protruding. In some species of hermit-crabs there is always to be found on the sh.e.l.l near the opening a sea-anemone. "This sea-anemone is carried from place to place by the crab, and in this way is much aided in obtaining food. On the other hand, the crab is protected from its enemies by the well-armed and dangerous tentacles of its companion. On the tentacles there are many thousand long slender stinging threads, and the fish that would eat the hermit-crab must first deal with the stinging anemone." If the sea-anemone be torn away from the sh.e.l.l the crab will wander about seeking another anemone. When he finds one, he struggles to loosen it from the rock to which it is attached, and does not rest until he has torn it loose and placed it on his sh.e.l.l.

In the case of the hermit-crab and the sea-anemone there is no doubt of the mutual advantage derived from their communal life. But this mutual advantage is not so obvious in some cases of commensalism, where indeed most or all of the advantage often seems to lie with one of the animals, while the other derives little or none, but on the other hand suffers no injury. For example, "small fish of the genus _Nomeus_ may often be found accompanying the beautiful Portuguese man-of-war (_Physalia_) as it sails slowly about on the ocean's surface. These little fish lurk underneath the float among the various hanging thread-like parts of the man-of-war which are provided with stinging cells. They are protected from their enemies by their proximity to these stinging threads, but of what advantage to the man-of-war their presence is is not understood." Similarly in the nests of the various species of ants and termites many different kinds of other insects have been found. "Some of these are harmful to their hosts, in that they feed on the food-stores gathered by the industrious and provident ant, but others appear to feed only on refuse or useless substances in the nest. Some may be of help to their hosts by acting as scavengers. Over one thousand species of these myrmecophilous (ant-loving) and termitophilous (termite-loving) insects have been recorded by collectors as living habitually in the nests of ants and termites."

=Parasitism.=--TECHNICAL NOTE.--As examples of temporary external parasites find and examine fleas and ticks on dogs and cats, red mites on house-flies and gra.s.shoppers (at the bases of the wings), etc. As examples of permanent external parasites find bird-lice on pigeons or domestic fowls or on other birds. Note the absence of wings and the peculiarly modified body shape of these parasites.

Examine a bird-louse under the microscope; note the absence of compound eyes (it has simple eyes) and absence of wings; note bits of feathers, its food, in stomach, showing through the body. Find, as examples of internal parasites, intestinal worms or flukes.

Examine trichinized pork to see _Trichinae_ in muscles. Examine preserved specimens of tapeworms. Collect pupae of some common b.u.t.terfly or moth and keep them in the schoolroom until either the b.u.t.terflies or ichneumon flies issue. Some will surely be parasitized, and yield ichneumon flies (parasites) instead of a b.u.t.terfly. As examples of degeneration by quiescence examine barnacles (found on outer rocks of seash.o.r.e at low tide; easily obtained as preserved specimens by inland schools) and the females of scale-insects. These insects may be found on oleanders (the black scale, _Lecanium oleae_) or fruit-trees (the San Jose scale, _Aspidiotus perniciosus_). Note the great degeneration of the adult female of the San Jose scale; it has no eyes, antennae, wings, or legs. The young may be found crawling about at certain times of the year; they have eyes, antennae and legs.

In addition to the various ways of living together among animals, already described, namely, the social and communal life of individuals of a single species and the commensal and symbiotic life of individuals of different species, there is another and very common kind of a.s.sociation among animals. This is the a.s.sociation of parasite and host; the a.s.sociation between two sorts of animals whereby one, the parasite, lives on or in the other, the host, and at the expense of the host. In this a.s.sociation the parasite gains advantages great or small, sometimes even obtaining all the necessities of life, while the host gains nothing, but suffers corresponding disadvantage, often even the loss of life itself. _Parasitism_ is a phenomenon common in all the large groups of animals, though the parasites themselves are mostly invertebrates. There are parasitic Protozoa, worms, crustaceans, insects, and molluscs, and a few vertebrates.

Some parasites, like the fleas and lice, live on the surface of the body of the host. These are called _external parasites_. Others, as the tapeworms, live exclusively inside the body; such are called _internal parasites_. Again, some, as the bird-lice, which are external parasites feeding on the feathers of birds, spend their whole lifetime on the host; they are called _permanent parasites_. Others, as a flea, which leaps on or off its host as caprice directs, or like certain parasites which as young live free and active lives, finally attaching themselves to some host and remaining fixed there for the rest of their lives, are called _temporary parasites_. Such a grouping is purely arbitrary and exists simply for the sake of convenience. It is not rigid, nor does it cla.s.s parasites in their proper natural groups.

When various parasites are examined it will be noted that practically in all cases the body of a parasite is simpler in structure than the body of other animals closely related to it; that is, species which live parasitically, obtaining their food from and being carried about by a host, have simpler bodies than related forms that live free active lives, competing for food with other animals about them. This simplicity is not primitive, but results from the loss or atrophy of the structures which the special mode of life of the parasite renders useless. Many parasites are attached firmly by hooks or suckers to their host, and do not move about independently of it. They have no need of the power of locomotion, and accordingly are usually without wings, legs, or other locomotory organs. Because they have no need of locomotion they have no need of organs of orientation, those special sense organs like the eyes, ears, and feelers which serve to guide and direct the moving animal; and most fixed parasites will be found to have no eyes, or any of those organs accessory to locomotion, and which serve for the detection of food or of enemies. Because these important organs, which depend for their successful activity on a well-organized nervous system, are lacking, the nervous system of parasites is usually very simple. Again, because the parasite usually feeds on the already digested food or the blood of its host, most parasites have a very simple alimentary ca.n.a.l, or even none at all. Finally, as the fixed parasite leads a wholly sedentary and inactive life, the breaking down and rebuilding of tissue in its body goes on very slowly and in minimum degree, so that there is little need of highly developed respiratory and circulatory systems; and most fixed and internal parasites have these systems of organs decidedly simplified. Altogether the body of a fixed permanent parasite is so simplified and so wanting in all those special structures which characterize the active, complex animals that it often presents a very different appearance from those forms with which we know it to be nearly related. This simplicity due to loss or reduction of parts is called _degeneration_. Such simplicity of body-structure due to degeneration is, however, essentially different in its origin from the simplicity of the lower simpler animals. In them the simplicity of body is primitive; they are generalized animals; the simplicity of degeneration is acquired; it is really an adaptation, or specialization.