to say that these unknown elements can hardly be considered as being in his environment. Hence it follows that man's discoveries are constantly changing in a real sense the world in which he lives. It is one world when he knows a few of the properties of wood and stone. It is another when he discovers the metals, learns to use copper, tin, zinc, gold, silver, iron, and yesterday platinum and aluminum, today radium.

There can be no greater mistake than to think of the physical environment as fixed and unchanging, though that mistake is far from rare. Geology was once taught as if the earth were completed ages ago. As a matter of fact, it is changing as rapidly today as ever, so far as we can tell. The formation of rocks and their gradual disintegration under the influence of snow, rain and wind is still going on. Change, eternal change, is the one great fact in nature.

Our evidence shows that the physical contour of the earth has been greatly altered in the course of time. We know that the Appalachian Mountains are but the stumps left of a great range. We know that there were four (possibly five) glacial epochs when the ice sheet covered North America as far south as Pennsylvania and Missouri. Sometime during this period the Great Lakes came into existence. The cause of these epochs, their duration, their disappearance and their effect upon organic life are some of the most fascinating puzzles of history. Oceans now exist where once were dry land and mountain ranges, whose slopes were once ocean beaches.

It is evident too that the climate of any given area of the earth has often undergone tremendous changes. Iceland was once a subtropical country with a flora resembling that of Florida, rather than its own. All the country recently

labeled as the Great American Desert was once a moist, fertile land, densely populated by animals. Now these great herds are gone and the Great Salt Lake is but a puddle in comparison to its ancient self when it had an outlet to the Pacific. It appears that west Central Asia has been drying up throughout all recorded time. Huntington says: "If it be proved that the climate of any region has changed during historic times, it follows that the nature of the geographic provinces concerned must have been altered more or less. For example, among the human inhabitants of Central Asia widespread poverty, want and depression have been substituted for comparative competence, prosperity and contentment. Disorder, wars and migrations have arisen. Race has been caused to mix with race under new physical conditions, which have given rise to new habits and character. The impulse toward change and migration received in the vast arid regions of Central Asia has spread outward and involved all Europe in the confusion of the Dark Ages. And more than this, the changes of climate which affected Central Asia were not confined to that region apparently, but extended over a large part of the inhabited earth." 32 In 1911 and 1912 Huntington had opportunity to examine some 450 big trees of California which varied in age from 230 to 3200 years. Eighty were over 2000 years old. Judging from the width of the rings the sequoia grew on the average of 30 per cent faster at the time of Christ than it did A.D. 1500.33 In our own arid southwest there are many evidences of an earlier civilization when large areas were cultivated both with and without irrigation. The Pimas call these people the Hohokam or "perished

32 HUNTINGTON, E. The Pulse of Asia, pp. 15, 16.

33 HUNTINGTON, E. American Historical Review, January, 1913.

ones." Mr. Huntington has recently made some most interesting studies in this country. Of the Santa Cruz Valley in New Mexico he writes: "The part of the valley which is now capable of cultivation contains ruins which indicate that all the available land was utilized in the past. Below the point where irrigation is now possible there are three large groups of ruins, and the three together must have had as many people as the higher regions where there is still water. In other words, it seems as if the Santa Cruz Valley once had at least twice as many people as it could at present support, and half of these lived where the white man could not now get a living from agriculture." 34

The question over which scientists are now puzzling is whether these and other changes come in cycles. There are many who believe that there is a periodicity in the sun spots and, correspondingly, periods of greater evaporation and rainfall on earth. The decision must be left to the future.

Certain other changes are also clear. The rivers of the world are building great alluvial deltas out of the material taken from the hills. Thus through the ages the Mississippi has made its delta from Cairo, Illinois, to its present mouth. It annually carries to the sea 225,000 acre feet of fertile soil and builds new land for future men. That this process may involve serious loss to existing society is also evident.

Now all these changes, whether the yearly cycle of temperature, or the greater pulsations as they have been called, involve constant readjustments of life. Sudden changes are always most dangerous. Even change in food and drink may produce serious results. There are many

34 HUNTINGTON, E. Report of Smithsonian Institute, 1912, p. 393.

small ponds and streams which occasionally go dry with great loss of life. Man prepares for the coming winter, but the earthquake, tidal wave or volcanic eruption finds him helpless. If the coal supply failed this year our civilization might easily perish. Realizing that at some period it will fail, our ingenuity may enable us to find some substitute. Tides, winds and sun rays will some day be valued sources of energy.

Against the minor changes of storm and frost man easily protects himself. Not so, however, the lower forms of life. Frost indeed simplifies man's problems by killing his insect pests. Winter in the colder regions destroys countless myriads of organisms. Some scheme of suspended animation as it were must be evolved if any are to survive. Nature is fertile in inventions. By means of roots, unaffected by freezing and seeds which lie on the ground and germinate the next year, she saves her plants. For the animals there are eggs to hatch the next year, larvæ to penetrate the earth, cocoons from which the new form comes, hibernation for bears, migration for birds, while every rotten log or hollow tree literally teems with dormant life.

"The animal lives in an environment which is constantly changing. Its spontaneous movements are constantly bringing it into different conditions. It tends to regulate its internal processes by selecting the point in the environment in which its internal processes are not disturbed." 35 The mollusca living in the tide lines must be small enough to find protection by creeping into crevices of the rocks unless, as is usually the case, they have a strong pedal sucker which enables them to remain fixed. The blood temperature of hibernating rodents 35 SHELFORD, V. E. o. c., p. 29.

falls to about 40°, sometimes nearly to 35°. A little oxygen reaches them, but so little is required that immersion of an hour in carbon dioxid will not kill them. They must be fat when they begin the long sleep and must waken gradually. It is thought that if their temperature falls too low they awaken automatically, and by movement and the inhalation of oxygen again raise their temperature. The differences in the behavior of frogs in hot and cold water is interesting. In the former they are extremely lively, and as the temperature drops they become sluggish and drowsy.

Animals show marked powers of readaptation - to avoid the use of the word reason which some feel is a prerogative of the human. Our common chimney swifts once nested in hollow trees or crevices in the rock, but they accept chimneys as satisfactory substitutes. The black-throated hunting and other ground-building birds will, if their nests are destroyed, frequently build late in the season in trees. Ducks and geese that are hunted become extremely wary, but will quickly discover and frequent in large numbers a lake where shooting is prohibited. In parts of Africa because of the constant hunting the buffalo feeds only at night.

Even the daily changes are significant. Sleep results apparently from a drugging of the system by the carbon. dioxid and other elements produced by everyday activity, which are less rapidly eliminated. At night, therefore, the eliminating process goes on till a balance is again secured. Possibly some structures are due to those daily changes. There is reason to believe that the barring of the feathers of some birds is due to the low blood pressure and poor circulation at night.

An interesting rhythm is shown by the phosphorescent