so it seems that its illumination is about 100 per cent efficient, as no light-producing energy appears to be lost.

Noctiluca, however, is not the most powerful light-producer for its size. That distinction belongs to another planktonic creature, the little ostracode crustacean Cypridena.

MAN HAVING CRUSTACEAN'S ILLUMINATING POWER COULD LIGHT CITY

The light from this animal is so strong that one part of its luminous gland in about two billion parts of water will impart a visible glow to this medium. If a human being possessed an organ giving the same proportionate volume and degree of illumination, he would be capable of lighting up the business area of a city like Washington, D. C.

Yet it seems that this light is of doubtful value not only to Cypridena, Noctiluca, and other animals, but to those higher creatures that possess it, such as certain insects. In the latter instances it is very probable that luminescence is merely an incidental manifestation of a more fundamental cause that has for an end not the production of light, but sexual attraction, or possibly something still far removed from our inquiring minds.

THE NOCTILUCA SEEMS TO BE IMMORTAL

It is hardly possible to speak of Noctiluca or, for that matter, of any other planktonic protozoan without adverting for a moment to its unique distinction in the world of living things as belonging to the only class of animals which are literally immortal. Noctiluca never dies from so-called natural causes, such as old age. In truth, except for accidental injury or other bodily harm, it lives forever, so to speak.

The secret of its immortality lies in its mode of reproduction. At a certain stage in the life of the individual a division of its body takes place. A constriction is first formed in the cell wall around the middle of the sphere, this constriction rapidly growing deeper and finally separating the animal into distinct and equal parts.

Each of these parts in turn becomes a smaller but exact duplicate of its single prototype. That is to say, what was orig

A CLOSE-UP OF THE HAUL

The cloudlike mass in the lower part of the test tube is composed of many thousands of tiny forms of plants and animals. Individually, the majority can barely be seen with the naked eye. Their concentration here in such numbers is the result of the tow net drawn through the water (see pages 300, 301, and 302).

inally one adult motner cell has resolved. itself into twin daughters.

When the twins grow older they likewise divide, and the cycle continues.

It should be observed, however, that this method of reproduction by fission, as it is termed, does not go on indefinitely without variation; for in some succeeding generation two individuals will meet, a fusion of their cell substance will occur, and the pair will appear to coalesce into a single unit, thus apparently reversing the process of division.

MOST PLANT LIFE OF THE OPEN OCEAN IS FLOATING AND MICROSCOPIC

Sooner or later the fused cells develop prominences or buds on the surface of the sphere. These ultimately become detached and swim away as zoospores, or young Noctilucas, who, when they grow

up, will again carry on reproduction by division in the manner first described.

When we turn to the microscopic plants. of the sea, we find that here, too, Nature has been lavish in bestowing her charms. Now the vegetation of the sea exists in far greater quantities than would appear to the casual observer of seaweeds along the shore. There are also stupendous numbers of plants, nearly all microscopic, that float free in the sea.

These small floating forms, with extremely rare exceptions, are the only kinds of vegetal life found in the open ocean. And here is a curious thing: unlike the plants of the land, wherein green is the predominant color, these of the open sea

are of various hues, but the great majority are brown; very few are green.

Yet whatever attractiveness they may lack in their coloring is more than compensated for by their exquisite and marvelous structures. A case in point is the Diatom (see Color Plate III). The form of this plant is quite variable. It may be disklike, or boat-shaped, or one of a dozen other patterns. It has a shell of silica which is made of two halves that fit together like the top and bottom of a pillbox. The plant lives within the shell. But what a wonderful shell!

The regular and geometrical arrangement of the embossing of these pretty patterns is nowhere else duplicated in Nature.

They form a veritable island of mountains in the Great Plains, and the natural attractiveness of their heavily wooded peaks and ridges and their well-watered valleys is made doubly appealing by the contrast of the surrounding country.

Touching the hills on the southeast is one of the most desolate yet interesting areas in America, the Big Bad Lands of southwestern South Dakota, where, through thousands of years, rains have carved the light clays and sands into fluted columns and cones, jagged buttresses, and a thousand other fantastic forms (see illustration, page 313).

THE “LITTLE ROCKIES" OF THE WEST

The Black Hills are not a part of the Rockies, but they may be looked upon geologically as dwarf brothers to those giant mountains, showing the family characteristics on a smaller scale.

Both probably were formed about the same time. Data assembled by geologists show that millions of years ago, during the Mesozoic Age, a vast sea washed over the entire central portion of the continent, covering the site of the Black Hills. It was at this time that the softer rocks of the region were formed as sediments. When, at the end of the Mesozoic, the great flow of melted granite welled up from below to raise the Rockies, it found a weak spot at the site of the Black Hills and rose there, too, poking the surface limestones and other rocks up as a rising tent-pole pushes up the canvas.

The cooking process which the surface rocks underwent at that time had much to do with making the Black Hills one of the richest mineral regions in the country.

In the ages since these mountains rose, the softer stones have been weathered away in many places, exposing the hard. granite, as at Harney Peak. Around the base of this peak stand great spires, rem

The Black Hills derived their name from the blue-black appearance of the dense pine forests when viewed from a distance.

They played a peculiar and interesting part in the frontier life of America. Before the coming of the white man these wooded uplands were one of the favorite hunting grounds of the Indians of the surrounding Plains. They were unsettled by whites long after emigrants had established themselves in California, Texas, Colorado, Utah, and other territories farther west. This was because all of western South Dakota was reserved for the Sioux Indians.

For generations this region of hills and trees and streams, off beyond the dry prairies, had been a sort of legendary promised land to people of the near West, and more than one party of adventurous emigrants had dared everything in a dash toward, it across the hostile Indian country. But most of these bold men fell victims to the avenging tomahawk.

What Indian tribe possessed this choice hunting ground of the West before the white man came to America is unknown; but since the opening of the West several distinct tribes have occupied the region, each in turn forced out by a stronger group. In all cases these newcomers have appeared from the east and almost without exception the older residents have moved farther west. Doubtless this parade of tribes through the Black Hills has been going on for many hundreds of years.

INDIAN TRIBES INVADE BLACK HILLS

The Crow Indians are believed to have been in possession of the Black Hills near the beginning of the eighteenth century. Later the Poncas entered the eastern portion of the highlands, but, finding them occupied, turned back eastward. The Cheyennes were the next successful invaders from the east, and were in possession of the territory when Lewis and Clark passed in 1804.

Next came the Sioux, the last of the red