AN EXAMPLE OF CLa.s.sIFICATION.

TECHNICAL NOTE.--There should be provided a small set of bird-skins which will serve just as well as freshly killed birds, and which may be used for successive cla.s.ses, thus doing away with the necessity of shooting birds. The birds suggested for use are among the commonest and most easily recognizable and obtainable. They may be found in any locality at any time of the year. The skins can be made by some boy interested in birds and acquainted with making skins, or by the teacher, or can be purchased from a naturalists'

supply store, or dealer in bird skins. The skins will cost about 25 cents each. This example or lesson in cla.s.sification can be given just as well of course with other species of birds, or with a set of some other kinds of animals, if the teacher prefers. Insects are especially available, b.u.t.terflies perhaps offering the most readily appreciated resemblances and differences.

=Species.=--Examine specimens of two male downy woodp.e.c.k.e.rs (the males have a scarlet band on the back of the head). (In the western States use Gardiner's downy woodp.e.c.k.e.r.) Note that the two birds are of the same size, have the same colors and markings, and are in all respects alike. They are of the same kind; simply two individuals of the same kind of animal. There are hosts of other individuals of this kind of bird, all alike. This one kind of animal is called a _species_. The species is the smallest[4] group recognized among animals. No attempt is made to distinguish among the different individuals of one kind or species of animal as we do in our own case.

Examine a specimen of the female downy woodp.e.c.k.e.r. It is like the male except that it does not have the scarlet neck-band. But despite this difference we know that it belongs to the same species as the male downy because they mate together and produce young woodp.e.c.k.e.rs, male and female, like themselves. There are thus two sorts of individuals,[5] male and female, comprised in each species of animal.

A _species_ is a group of animals comprising similar individuals which produce new individuals of the same kind usually after the mating together of individuals of two s.e.xes which may differ somewhat in appearance and structure.

Examine a male hairy woodp.e.c.k.e.r and a female; (in western States subst.i.tute a Harris's hairy woodp.e.c.k.e.r). Note the similarity in markings and structure to the downy. Note the marked difference in size. Make notes of measurements, colors and markings, and drawings of bill and feet, showing the resemblances and the differences between the downy woodp.e.c.k.e.r and the hairy woodp.e.c.k.e.r. These two kinds of woodp.e.c.k.e.rs are very much alike, but the hairy woodp.e.c.k.e.rs are always much larger (nearly a half) than the downy woodp.e.c.k.e.rs and the two kinds never mate together. The hairy woodp.e.c.k.e.rs const.i.tute another species of bird.

=Genus.=--Examine now a flicker (the yellow-shafted or golden-winged flicker in the East, the red-shafted flicker in the West). Compare it with the downy woodp.e.c.k.e.r and the hairy woodp.e.c.k.e.r. Make notes referring to the differences, also the resemblances. The flicker is very differently marked and colored and is also much larger than the downy woodp.e.c.k.e.r, but its bill and feet and general make-up are similar and it is obviously a "woodp.e.c.k.e.r." It is, however, evidently another species of woodp.e.c.k.e.r, and a species which differs from either the downy or the hairy woodp.e.c.k.e.r much more than these two species differ from each other. There are two other species of flickers in North America which, although different from the yellow-shafted flicker, yet resemble it much more than they do the downy and hairy woodp.e.c.k.e.rs or any other woodp.e.c.k.e.rs. We can obviously make two groups of our woodp.e.c.k.e.rs so far studied, putting the downy and hairy woodp.e.c.k.e.rs (together with half a dozen other species very much like them) into one group and the three flickers together into another group. Each of these groups is called a _genus_, and genus is thus the name of the next group above the species. A genus usually includes several, or if there be such, many, similar species. Sometimes it includes but a single known species. That is, a species may not have any other species resembling it sufficiently to group with it, and so it const.i.tutes a genus by itself. If later naturalists should find other species resembling it they would put these new species into the genus with the solitary species. Each genus of animals is given a Greek or Latin name, of a single word. Thus the genus including the hairy and downy woodp.e.c.k.e.rs is called _Dryobates_; and the genus including the flickers is called _Colaptes_. But it is necessary to distinguish the various species which compose the genus _Colaptes_, and so each species is given a name which is composed of two words, first the word which is the name of the genus to which it belongs, and, second, a word which may be called the species word. The species word of the Yellow-shafted Flicker is _auratus_ (the Latin word for golden), so that its scientific name is _Colaptes auratus_. The natural question, Why not have a single word for the name of each species? may be answered thus: There are already known more than 500,000 distinct species of living animals; it is certain that there are no less than several millions of species of living animals; new species are being found, described and named constantly; with all the possible ingenuity of the word-makers it would be an extremely difficult task to find or to build up enough words to give each of these species a separate name. This is not attempted. The same species word is often used for several different species of animals, but never for more than one species belonging to a given genus. And the names of the genera are never duplicated. (There are, of course, much fewer genera than species, and the difficulty of finding words for them is not so serious.) Thus the genus word in the two-word name of a species indicates at once to just what particular genus in the whole animal kingdom the species belongs, while the second or species word distinguishes it from the few or many other species which are included in the same genus. This manner of naming species of animals and plants (for plants are given their scientific names according to the same plan) was devised by the great Swedish naturalist Linnaeus in the middle of the eighteenth century and has been in use ever since.

=Family.=--Examine a red-headed woodp.e.c.k.e.r (_Melanerpes erythrocephalus_) and a sapsucker (_Sphyrapicus varius_) and any other kinds of woodp.e.c.k.e.rs which can be got. Find out in what ways the hairy and downy woodp.e.c.k.e.rs (genus _Dryobates_), the flickers (genus _Colaptes_) and the other woodp.e.c.k.e.rs resemble each other. Examine especially the bill, feet, wings and tail. These birds differ in size, color and markings, but they are obviously all alike in certain important structural respects. We recognize them all as woodp.e.c.k.e.rs.

We can group all the woodp.e.c.k.e.rs together, including several different genera, to form a group which is called a _family_. A family is a group of genera which have a considerable number of common structural features. Each family is given a proper name consisting of a single word. The family of woodp.e.c.k.e.rs is named _Picidae_.

We have already learned that resemblances between animals indicate (usually) relationship, and that cla.s.sifying animals is simply expressing or indicating these relationships. When we group several species together to form a genus we indicate that these species are closely related. And similarly a family is a group of related genera.

=Order.=--There are other groups[6] higher or more comprehensive than families, but the principle on which they are const.i.tuted is exactly the same as that already explained. Thus a number of related families are grouped together to form an _order_. All the fowl-like birds, including the families of pheasants, turkeys, grouse and quail, all obviously related, const.i.tute the order of gallinaceous birds called _Gallinae_. The families of vultures, hawks and owls const.i.tute the order of birds of prey, the _Raptores_, and the families of the thrushes, wrens, warblers, sparrows, black-birds, and many others const.i.tute the great order of perching birds (including all the singing birds) called the _Pa.s.seres_.

=Cla.s.s and branch.=--But it is evident that all of these orders, together with the other bird orders, ought to be combined into a great group, which shall include all the birds, as distinguished from all other animals, as the fishes, insects, etc. Such a group of related orders is called a _cla.s.s_. The cla.s.s of birds is named _Aves_. There is a cla.s.s of fishes, _Pisces_, and one of frogs and salamanders, _Batrachia_, one of snakes and lizards called _Reptilia_, and one of the quadrupeds which give milk to their young called _Mammalia_. Each of these cla.s.ses is composed of several orders, each of which includes several families and so on down. But these five cla.s.ses of Pisces, Batrachia, Reptilia, Aves and Mammals agree in being composed of animals which have a backbone or a backbone-like structure, while there are many other animals which do not have a backbone, such as the insects, the starfishes, etc. Hence these five backboned cla.s.ses may be brought together into a higher group called a _branch_ or _phylum_.

They compose the branch of backboned animals, the branch _Vertebrata_; all the animals like the starfishes, sea-urchins and sea-lilies which have the parts of their body arranged in a radiate manner compose the branch _Echinodermata_; all the animals like the insects and spiders and centipedes and crabs and crayfishes which have the body composed of a series of segments or rings and have legs or appendages each composed of a series of joints or segments make up the branch _Arthropoda_. And so might be enumerated all the great branches or princ.i.p.al groups into which the animal kingdom is divided.

In the remainder of this book the cla.s.sification of animals is always kept in sight, and the student will see the terms species, genus, family, order, etc., practically used. In it all should be kept constantly in mind the significance of cla.s.sification, that is, the existence of actual relationships among animals through descent.

FOOTNOTES:

[4] The lesser group called _variety_, or subspecies, we may leave out of consideration for the present.

[5] Some species of animals are not represented by male individuals: and in some all the individuals are hermaphrodites, as explained in chapter XIV.

[6] Each of these higher groups has a proper name composed of a single word. In the case of no group except the species is a name-word ever duplicated. Each genus, family, order, or higher group has a name-word peculiar to it, and belonging to it alone.

CHAPTER XV

BRANCH PROTOZOA: THE ONE-CELLED ANIMALS

Of this group the structure and life-history of the Amba (_Amba_ sp.) and the Slipper Animalcule (_Paramcium_ sp.) have already been treated in Chapter VI. Another example is the

BELL ANIMALCULE (_Vorticella_ sp.)

TECHNICAL NOTE.--Specimens of _Vorticella_ may usually be found in the same water with _Amba_ and _Paramcium_. The individuals live together in colonies, a single colony appearing to the naked eye as a tiny whitish mould-like tuft or spot on the surface of some leaf or stem or root in the water. Touch such a spot with a needle, and if it is a Vorticellid colony it will contract instantly. Bring bits of leaves, stems, etc., bearing Vorticellid colonies into the laboratory and keep in a small stagnant-water aquarium (a battery-jar of pond-water will do).

Examine a colony of _Vorticella_ in a watch-gla.s.s of water or in a drop of water on a gla.s.s slide under the microscope. Note the stemmed bell-shaped bodies which compose the colony. Each bell and stem together form an individual _Vorticella_ (fig. 8.) How are the members of the colony fastened together? Tap the slide and note the sudden contraction of the animals; also the details of contraction in the case of an individual. Watch the colony expand; note the details of this movement in the case of an individual.

Make drawings showing the colony expanded and contracted.

With higher power examine a single individual. Note the thickened, bent-out, upper margin of the bell. This margin is called the _peristome_. With what is it fringed? The free end of the bell is nearly filled by a central disk, the _epistome_, with arched upper surface and a circlet of _cilia_. Between the epistome and peristome is a groove, the _mouth_ or _vestibule_, which leads into the body.

Study the internal structure of the transparent, bell-shaped body.

Note the differentiation of the protoplasm comprising the body into an inner transparent colorless _endosarc_ containing various dark-colored granules, vacuoles, oil-drops, etc., and an outer uniformly granular _ectosarc_ not containing vacuoles. Is the stalk formed of ectosarc or endosarc or of both? Note the curved _nucleus_ lying in the endosarc.

(This may be difficult to distinguish in some specimens.) Note the numerous large circular granules, the _food vacuoles_. Note the _contractile vesicle_, larger and clearer than the food vacuoles. Note the thin _cuticle_ lining the whole body externally. A high magnification will show fine transverse ridges or rows of dots on the cuticle.

[Ill.u.s.tration: FIG. 8.--_Vorticella_ sp.; one individual with stalk coiled, and one with stalk extended. (From life.)]

Make a drawing showing the internal structure.

Observe a living specimen carefully for some time to determine all of its movements. Note the contraction and extension of the stalk, the movements of the cilia of peristome and epistome, the flowing or streaming of the fluid endosarc (indicated by the movements of the food vacuoles), the behavior of the contractile vesicle.

Make notes and drawings explaining these motions.

Specimens of _Vorticella_ may perhaps be found dividing, or two bell-shaped bodies may be found on a single stem, one of the bodies being sometimes smaller than the other. These two bodies have been produced by the longitudinal division or fission of a single body. In this process a cleft first appears at the distal end of the bell-shaped body, and gradually deepens until the original body is divided quite in two. The stalk divides for a very short distance. One of the new bell-shaped bodies develops a circlet of cilia near the stalked end. After a while it breaks away and swims about by means of this basal circlet of cilia. Later it settles down, becomes attached by its basal end, loses its basal cilia and develops a stalk.

"Conjugation occurs sometimes, but it is unlike the conjugation of _Paramcium_ in two important points: Firstly, the conjugation is between two dissimilar forms; an ordinary large-stalked form, and a much smaller free-swimming form which has originated by repeated division of a large form. Secondly, the union of the two is a complete and permanent fusion, the smaller being absorbed into the larger. This permanent fusion of a small active cell with a relatively large fixed cell, followed by division of the fused ma.s.s, presents a striking a.n.a.logy to the process of s.e.xual reproduction occurring in higher animals."

OTHER PROTOZOA

Besides the _Amba_, _Paramcium_, and _Vorticella_ there are thousands of other Protozoa. Most of them live in water, but a few live in damp sand or moss, and some live inside the bodies of other animals as parasites. Of those which live in water some are marine, while others are found only in fresh-water streams and lakes.

[Ill.u.s.tration: FIG. 9.--Sun animalcule, a fresh-water protozoan with a siliceous skeleton, and long thread-like protoplasmic prolongations.

(From life.)]

=Form of body.=--The Protozoa all agree in having the body composed for its whole lifetime of a single cell,[7] but they differ much in shape and appearance. Some of them are of the general shape and character of _Amba_, sending out and retracting blunt, finger-like pseudopodia, the body-ma.s.s itself having no fixed form or outline but constantly changing. Others have the body of definite form, spherical, elliptical, or flattened, enclosed by a thin cuticle, and having a definite number of fine thread-like or hair-like protoplasmic prolongations called flagella or cilia. Many of the familiar Protozoa of the fresh-water ponds always have two whiplash-like flagella projecting from one end of the body. By means of the lashing of these flagella in the water the tiny creature swims about. Others have many hundreds of fine short cilia scattered, sometimes in regular rows, over the body-surface. The Protozoan swims by the vibration of these cilia in the water.

[Ill.u.s.tration: FIG. 10.--_Stentor_ sp.; a protozoan which may be fixed, like _Vorticella_, or free-swimming, at will, and which has the nucleus in the shape of a string or chain of bead-like bodies. The figure shows a single individual as it appeared when fixed, with elongate, stalked body, and as it appeared when swimming about with contracted body. (From life.)]

There is no stagnant pool, no water standing exposed in watering-trough or barrel which does not contain thousands of individuals of the one-celled animals. And in any such stagnant water there may always be found several or many different kinds or species. A drop of this water examined with the compound microscope will prove to be a tiny world (all an ocean) with most of its animals and plants one-celled in structure. A few many-celled animals will be found in it preying on the one-celled ones. There are sudden and violent deaths here, and births (by fission of the parent) and active locomotion and food-getting and growth and all of the businesses and functions of life which we are accustomed to see in the more familiar world of larger animals.

=Marine Protozoa.=--One usually thinks of the ocean as the home of the whales and the seals and the sea-lions, and of the countless fishes, the cod, and the herring, and the mackerel. Those who have been on the seash.o.r.e will recall the sea-urchins and starfishes and the sea-anemones which live in the tide-pools. On the beach there are the innumerable sh.e.l.ls, too, each representing an animal which has lived in the ocean. But more abundant than all of these, and in one way more important than all, are the myriads of the marine Protozoa.

Although the water at the surface of the ocean appears clear and on superficial examination seems to contain no animals, yet in certain parts of the ocean (especially in the southern seas) a microscopical examination of this water shows it to be swarming with Protozoa. And not only is the water just at the surface inhabited by one-celled animals, but they can be found in all the water from the surface to a great depth below it. In a pint of this ocean-water there may be millions of these minute animals. In the oceans of the world the number of them is inconceivable. And it is necessary that these Protozoa exist in such great numbers, for they and the marine one-celled plants (Protophyta) supply directly or indirectly the food for all the other animals of the ocean.

Among all these ocean Protozoa none are more interesting than those belonging to the two orders Foraminifera (fig. 11) and Radiolaria. The many kinds belonging to these orders secrete a tiny sh.e.l.l (of lime in the Foraminifera, of silica in the Radiolaria) which encloses most of the one-celled body. These minute sh.e.l.ls present a great variety of shape and pattern, many being of the most exquisite symmetry and beauty. The sh.e.l.ls are perforated by many small holes through which project long, delicate, protoplasmic pseudopodia. These fine pseudopodia often interlace and fuse when they touch each other, thus forming a sort of protoplasmic network outside of the sh.e.l.l. In some cases there is a complete layer of protoplasm--part of the body protoplasm of the Protozoan--surrounding the cell externally.

[Ill.u.s.tration: FIG. 11.--_Rosalina varians_, a marine protozoan (Foraminifera) with calcareous sh.e.l.l. (After Schultze.)]

When these tiny animals die their hard sh.e.l.ls sink to the bottom of the ocean, and acc.u.mulate slowly, in inconceivable numbers, until they form a thick bed on the ocean floor. Large areas of the bottom of the Atlantic Ocean are covered with this slimy ooze, called Foraminifera ooze or Radiolaria ooze, depending on the kinds of animals which have formed it. Nor is it only in present times that there has been a forming of such beds by the marine Protozoa. All over the world there are thick rock strata composed almost exclusively of the fossil sh.e.l.ls of these simplest animals. The chalk-beds and cliffs of England, and of France, Greece, Spain, and America, were made by Foraminifera.

Where now is land were once oceans the bottoms of which have been gradually lifted above the water's surface. Similarly the rock called Tripoli found in Sicily and the Barbadoes earth from the island of Barbadoes are composed of the sh.e.l.ls of ancient Radiolaria.

It is thus evident that the Protozoa is an ancient group of animals.

As a matter of fact zoologists are certain that it is the most ancient of all animal groups. All of the animals of the ocean depend upon the marine Protozoa and the marine Protophyta, one-celled plants, for food. Either they feed on them directly, or prey on animals which in turn prey on these simplest organisms. A well-known zoologist has said: "The food-supply of marine animals consists of a few species of microscopic organisms which are inexhaustible and the only source of food for all the inhabitants of the ocean. The supply is primeval as well as inexhaustible, and all the life of the ocean has gradually taken shape in direct dependence on it." The marine Protozoa are the only animals which live independently; they alone can live or could have lived in earlier ages without depending on other animals. They must therefore be the oldest of marine animals. By oldest is meant that their kind appeared earliest in the history of the world, and as it is certain that ocean life is older than terrestrial life--that is, that the first animals lived in the ocean--it is obvious that the marine Protozoa are the most ancient of all animal groups.

As already learned in the examination of examples of one-celled animals, it is evident that life may be successfully maintained without a complex body composed of many organs performing their functions in a specialized way. The marine Protozoa ill.u.s.trate this fact admirably. Despite their lack of special organs and their primitive way of performing the life-processes, that they live successfully is shown by their existence in such extraordinary numbers. They outnumber all other animals. The conditions of life in the surface-waters of the ocean are easy and constant, and a simple structure and simple method of performing the necessary life-processes are wholly adequate for successful life under these conditions.