In order that the myriad forms of animals may be studied with facility some system of classification is necessary. And now that we have learned that there exists a blood-relationship between the different kinds of animals. that system which most clearly expresses this relationship is doubtless the best. This system is termed the Natural Classification.

It is now generally believed that long ago, in early geological times, there existed on the earth only very simple animals and plants; and that from these simple beginnings more and more complex forms have been developed. This growth in complexity has taken place in different descendants of these simple primitive beings in very different ways, Thus while it is probable that the first animals lived in water, and very, many still do so, others have become adapted to life on the land, and in still others organs have been developed by which they can fly through the air and under each of these conditions we find a great diversity of forms, each fitted for some special mode of life.

The diversity of forms of animal life is much greater than is commonly supposed. A competent authority has estimated that there are now living on the earth more than one million species of animals. And these are merely the surviving descendants of immense series of beings that have existed in past geological times, the remaining tips of a great genealogical tree, of which many twigs and branches have perished.

The common figurative use of the word tree in this connection expresses well the convergence of the lines of descent toward the common ancestor from which existing forms have descended. But in one respect it may be misleading. If an ordinary tree be examined, the tip of one branch will closely resemble that of any other branch of the same tree. But in this figurative genealogical tree we must imagine a very different state of affairs. Here the law of growth is constant change; each branch grows in its own individual way; and each twig of each branch bears fruit peculiar to itself. The changes, however, are gradual; and thus the tips of closely-connected twigs will be similar though not identical; while the tips of two branches that separated early in the growth of the tree will be very different.

It is the effort of the systematist, one who studies the classification of animals and plants, to work out the relations which exist between the various tips of the genealogical tree. This study when carried to its fullest extent includes not only the study of existing forms of life, but also the study of those that have perished, the trunk-forms from which existing forms have descended. This, however, is a very difficult matter; and as yet only the beginnings of the Natural Classification have been made, see pp. 139 to 204.

If we accept this theory of descent, now almost universally accepted by naturalists, it is evident that when we take into account all the forms of life that have existed we cannot classify animals into well-marked groups; for as the modification in form is gradual, series of connecting links have existed between any two forms that might be selected.

But practically the student that confines his attention to the study of living forms can classify these forms into more or less well-marked groups, for many of the connecting links have perished; in fact, the groups of living animals and plants are so distinct that it is only in recent years that naturalists have come to understand the blood-relationship referred to above.

We find that the Animal and Vegetable Kingdoms are made up of a vast assemblage of individuals, each the offspring of parents similar to itself, and each in turn producing similar offspring. Although the offspring is never exactly like either parent, the degree of variation in a single generation is slight. And thus we find that there exist large numbers of individuals which very closely resemble each other. Such .a collection of individuals is termed in popular language a kind, in scientific language a species. Thus the kind of pine trees known as pitch-pine is a species; and scrub-pine, still another. In the same way the name sparrow-hawk indicates a kind or species of hawk; and pigeon-hawk, another species.

Roughly speaking, a species is a collection of individuals which resemble each other as closely as the offspring of a single parent. For example, if any two pitch-pines be studied, nothing will be found to indicate that they may not have sprung from seeds grown upon the same tree. On the other hand, if a pitch-pine and a white-pine be carefully compared, they will be found so different that no competent observer would believe that they had a common parent.

Unfortunately this mode of defining the limits of a species cannot be depended upon. Many instances are known where forms of animals or plants living in widely-separated regions differ so greatly that they have been considered distinct species until more extended collections in the intermediate regions have brought to light series of intermediate forms. which connect the two so-called species so closely that it is impossible to say where the one ends and the other begins.

The only definite way of determining whether two forms are specifically distinct is to determine whether they naturally interbreed or not. We find among wild animals a sort of race prejudice which keeps the members of different species from pairing; although they may do so when demoralized by domestication. Except in the case of very-closely-allied species, the pairing of individuals of different species results in no offspring or in the production of sterile offspring.

This grouping of individuals into species not only facilitates our study of Natural History. but expresses certain important facts of inheritance and reproduction. A second and somewhat similar step is made by grouping species into genera.

We find that there exist groups of closely-allied species, species that resemble each other in all of the more important characters, and differ among themselves only in what are known as the specific characters. Such a group of species is termed a genus. Thus all the different species of pine taken together constitute the genus pine, or Pinus, as it is termed by botanists. There are many species of oak, as red-oak. live-oak. and water-oak. All of the species of oak taken together constitute the genus Quercus of botanists. Several species of hawks and falcons are classed together by zoologists as the genus Falco.

The genera in turn are grouped into families. Thus the pines. the spruces, and the larches resemble each other quite closely, and are classed together as the Pine Family (Abietineae); the falcons, hawks, kites, and eagles are classed together as the Falcon Family (Falconidae).

Closely-allied families are grouped together to form orders. The Pine Family, the Cypress Family, and the Yew Family comprise the Order Coniferae, or cone-bearing plants, of botanists. The Owl Family (Strigidae), the Falcon Family (Falconidae), and the Vulture Family (Vulturidae) constitute the Order Raptores, or Birds of Prey.

Closely-allied orders are grouped together to form classes. Thus all the orders of birds taken together constitute the Class Aves or Birds.

The classes are grouped into branches, which are the principal divisions of the Animal Kingdom. In studying the different forms of animals it is found that there are several distinct types of structure. Some animals are built upon one plan or structure, and others on other plans. All animals built on the same plan are said to belong to the same Branch. Thus the back-boned animals comprise the Branch Vertebrata; the clams. oysters, snails, cuttle-fish, and certain other allied forms comprise the Branch Molusca; and the insects, spiders, centipedes, lobsters, and their near relatives comprise the Branch Arthropoda.

All the branches of animals taken together constitute the Animal Kingdom.

It is not possible to lay down rules by which these different groups of animals can be limited. For, as has been shown in our discussion of species, all have been connected in past time by intermediate forms. But notwithstanding this, each of the terms given above (Branch, Class, Order, Family, Genus, and Species) expresses a pretty definite conception, which the student will learn to comprehend by practice in classifying animals. But the sequence in rank of these groups should be learned at the outset. Beginning with the most comprehensive it is as follows:

The principal divisions of the Vegetable Kingdom are not termed Branches; hence we will not make further use of botanical illustrations in this connection.

Animal Kingdom.
  Branch or Subkingdom.
    Class,
      Order,
        Family,
          Genus,
            Species,
              Individual.

It is sometime desirable to indicate other groups than those named above. Thus a family may be divided into subfamilies, or an order into suborders. And occasionally an even more minute division is made. Thus several closely-allied families may be grouped together as a superfamily, a group of lower rank than a suborder. The following table includes all the grades of groups now commonly employed:

Kingdom.
  Branch or Subkingdom.
    Class,
      Subclass.
        Superorder.
          Order.
            Suborder.
              Superfamily.
                Family.
                  Subfamily.
                    Genus.
                      Subgenus.
                        Species.
                          Subspecies.
                            Variety.
                              Individual.

II. Zoological Nomenclature.

At the beginning of his studies of Natural History the student is met with what is to him a new and strange set of names. These names are often long. In form they belong to a dead language, with which, in these days, even many educated people are unfamiliar. It is not strange that we often hear complaint respecting the difficulty of this nomenclature.

A little study of the matter, however, is sufficient to show the necessity for scientific names. The common names of animals will not answer our purpose; for the same name is often applied to widely different animals in different localities, while a single species of animal is known by totally different names in different sections of the country

In order that information respecting animals may be recorded so that there need not be any doubt regarding the animal to which reference is made, it is necessary that each species or group of species should have a distinct name by which it shall be known by naturalists in all parts of the world. Therefore, to each branch, class, order, family, genus, and species which has been described there has been given a special name, by which it is known, and which pertains to this group alone.

As this nomenclature is used by all naturalists of whatever nationality, it is necessary that the names should be in a language which can be understood by all. As Latin was the language in which most scientific books were written at the time this nomenclature was established, that language was chosen as the universal language of science; and the rule bas been adopted that all names of animals and plants shall be Latin, or Latin in form.

A little study of the matter, however, is sufficient to show the necessity for scientific names. The common names of animals will not answer our purpose; for the same name is often applied to widely different animals in different localities, while a single species of animal is known by totally different names in different sections of the country.

In order that information respecting animals may be recorded so that there need not be any doubt regarding the animal to which reference is made, it is necessary that each species or group of species should have a distinct name by which it shall be known by naturalists in all parts of the world. Therefore, to each branch, class, order, family, genus, and species which has been described there has been given a special name, by which it is known, and which pertains to this group alone.

As this nomenclature is used by all naturalists of whatever nationality, it is necessary that the names should be in a language which can be understood by all. As Latin was the language in which most scientific books were written at the time this nomenclature was established, that language was chosen as the universal language of science; and the rule has been adopted that all names of animals and plants shall be Latin, or Latin in form.

The name of a species consists of two words--the name of the genus to which the species belongs, followed by an adjective indicating the particular species; for in Latin an adjective follows the noun which it qualifies, instead of preceding it as in English. Thus the scientific name of the Pigeon-hawk is Falco columbarius; that of the Sparrow-hawk is Falco sparverius; and that of the Prairie-falcon is Falco mexicanus.

In the case of many species we find well-marked subspecies or geographical races which it is desirable to distinguish by name. Thus the Pigeon-hawk occurs over the whole of North America. But we California to Sitka, constitute a distinct geographical race known as the Black Merlin. As the Black Merlin and the typical Pigeon-hawk intergrade, they constitute a single species, which is known as Falco columbarius. To the Black Merlin has been applied the subspecific name suckleyi. When, therefore, it is desired to refer to the Black Merlin as distinguished from the typical Pigeon-hawk the term Falco columbarius suckleyi is used. If reference is to be made to the typical Pigeon-hawk as distinguished from the Black Merlin, it is designated as Falco columbarius columbarius.

In writing long names like those given above they are frequently abbreviated if the context is such that the abbreviations will be readily understood. Thus the name of the Black Merlin may be written Falco c. suckleyi or F. c. suckleyi.

Subspecific names are used by entomologists not only to distinguish geographical races, but also to distinguish the different forms of dimorphic and polymorphic species. A good illustration is afforded by a certain species of Swallow-tail Butterfly common in the Atlantic States. This species exists under two distinct forms; one of these is yellow marked with black, and has long been known as Jasoniades turnus; the other is almost entirely black, and has been known as Jasoniades glaucus. At first it was supposed that these were different species; but in recent years the two forms have been bred from eggs laid by the same female. It is thus evident that the two forms represent a single species. And as the form glaucus was first described its name is given to the species, which is now known as Jasoniades glaucus. This name Jasoniades glaucus is used when reference is made to the species as a whole. But if one wishes to refer to the black form alone, it is distinguished as Jasoniades glaucusglaucus; while the yellow form is distinguished as Jasoniades glaucus turnus.

In the illustrations just given the dimorphism occurs in the same generation. But many instances are known where the dimorphism is seasonal. Thus in the case of certain insects which pass through two or more generations in the course of a year, the different generations, or some of them, differ markedly in form or coloring from the others. These differences in many cases are so great that the different generations of the same species were believed to be distinct species till they were bred from each other. It is therefore often desirable to distinguish these different forms by subspecific names. Thus Iphiclides ajax is a species of Swallowtail Butterfly which exists under three distinct seasonal forms: an early spring form, I. ajax marcellus; a late spring form, I. ajax telamonides; and a summer form, I. ajax.

The name of a genus or of a subgenus is always a single word, and should be a noun in the singular number and nominative case.

The names of all groups of genera (i.e., families, orders, classes, and branches) consist each of a single word; and this word should be a plural noun in the nominative case.

The following practices regarding the forms of zoological names are now almost universally followed:

The names of all groups in zoology, from kingdom to subgenus inclusive, are written and printed with a capital initial letter.

Specific and subspecific names are written and printed with a small initial letter. Thus in writing the name of a species the generic name is capitalized, the specific name not; e.g., Iphiclides ajax.

The names of families end in idae,; the names of subfamilies. in inae.

It will aid the student greatly in the pronunciation of family and subfamily names to know that the i of -ida, in family names is short. and consequently the accent falls on the syllable preceding this letter; while the i olf -inae of subfamily names is long, and is consequently accented.* Numerous examples are given in the following pages.

* This in accordance with the rule of Latin grammar that in words of more than two syllables the penult if long is accented; but if the penult is short the accent falls on the antepeuult.