FIG. 5.-Three species of jack-rabbits, differing in size, color, and markings, but believed to be derived from a common stock. The differences have arisen through isolation and adaptation. The upper figure shows the head and fore legs of the black jack-rabbit (Lepus insularis), of Espiritu Santo Island, Gulf of California; the lower right-hand figure, the Arizona jack-rabbit (Lepus alleni), specimen from Fort Lowell, Arizona; and the lower left-hand figure is the San Pedro Martir jack-rabbit (Lepus martirensis), from San Pedro Martir, Baja California.

some question as to the number of these different types, but we are not far out of the way in recognizing seven principal ones. These give rise to the seven principal branches of the animal kingdom: Protozoa, Colenterata, Mollusca, Vermes, Arthropoda, Echinodermata, and Chordata (which includes vertebrates). The followers of Cuvier and Agassiz reduced these to four or five: Protozoa, Radiata, Mollusca, Articulata, and Vertebrata; but a more thorough knowledge of the different groups makes the larger number preferable, the radiates and the articulates being each divided into two. Many zoologists break up the Vermes into several distinct branches.

The branches are again divided into classes. Thus the mammals, birds, reptiles, amphibians, fishes, lampreys, and lancelets are classes of vertebrates. The insects form a class of Arthropods.

Each class is again divided into orders. The Glires or rodents, the gnawing animals, of which squirrels, mice, and rabbits are examples, form an order of mammals. The hoofed animals, Ungulata, form another, and each of these again contains many families.

Intermediate divisions are sometimes recognized, with the prefixes super and sub. A subfamily is a division of a family including certain genera. A superfamily is a group of related families within the limits of an order.

The red squirrel belongs to the branch Chordata, class Mammalia, order Glires, family Sciuridæ, genus Sciurus, species Hudsonicus.

20. Trinomial names.-Trinomial names are those in which the binomial name of a species is followed by a second adjective. These indicate subspecies or varieties connected with geographical distribution. Thus many forms have a northern variety, a southern variety, one in the mountains, one on the plains, in the forests, or in other peculiar situations.

Thus the gray squirrel, typically southern, has a sub

FIG. 6-Some chipmunks of California, showing distinct species produced through isolation. From nature, by WILLIAM SACKSTON ATKINSON.

species, Sciurus carolinensis leucotis (white-eared), in the Northern States, larger than the true Sciurus carolinensis, with the dark band on the back narrower. In Minnesota is another subspecies, Sciurus carolinensis hypophæus, with only a narrow streak of white on the belly. As animals come to be better known we can recognize by name more and more of these subspecies or geographical variations.

Even in the same locality the members of a species vary more or less, no two being exactly alike. The name variety is applied to any sort of variation which can be recognized. Usually varieties not having definite geographical range receive no scientific name. When forms in different geographical areas are found to intergrade or mix with one another they are known as subspecies, the one first named being regarded as the original species. When they do not intergrade they are called distinct species. The subspecies differ from the species in degree only. When the range of a species is crossed by an impassable barrier, the subspecies on either side of the barrier usually becomes in time a distinct species. Thus distinct species are said to be produced through isolation. The plates which follow may serve as illustrations of species and subspecies thus formed.

CHAPTER III

THE SIMPLEST ANIMALS OR PROTOZOA

21. Single-celled and many-celled animals. In almost every portion of the globe there are multitudes of animals whose body consists of but a single cell; while those forms more familiar to us, and usually of comparatively large size and higher development, such as sponges, insects, fishes, birds, and man himself, are composed of a multitude of cells. For this reason the animal kingdom has been divided into two great subdivisions, the Protozoa including all unicellular forms and the Metazoa embracing those of many cells.

22. Single-celled animals.-The division of the Protozoa comprises a host of animals, usually of microscopic size, inhabiting fresh or salt water or damp localities on land in nearly every portion of the globe. The greater number wage their little, though fierce, wars on one another without attracting much attention; others, in the sharp struggle, have been compelled to live upon or within the bodies of other animals, and many have become notorious because of the diseases they produce under such circumstances. few are in large measure responsible for the phosphorescence of the sea; and still others have long been favorite objects of study because of their marvelous beauty. Adapted for living under diverse conditions, the bodily form differs greatly, and yet all conform to three or four principal types, of which we may gain a good idea from the study of a few representative forms.