vegetation. Stone cutters, coal miners, painters and men who work in the dust of streets breathe in so much foreign matter that their lungs are coated, the supply of oxygen thereby reduced and they are made susceptible to the attacks of germ diseases, if not actually poisoned by the substances inhaled, as in the case of phosphorus or lead.

In the ocean and lakes there appears to be a pretty definite stratification of life, certain types being always found near the surface, others at lower depths while some are found only far below the surface. This is shown by the following table:

Stratification of Fish in Lake Michigan

Lake Herring (Argyrosomus artedi)

Whitefish (Coregonus clupeiformis) 21-36 meters; spawning 1519 meters usually

Lake trout (Cristivomer namaycush) below 25 meters, except during breeding season (2-25 meters)

The Long-jaw Whitefish (Argyrosomus prognathus) 36-66 meters The Blackfish (Argyrosomus nigripinnis) 70-80, rising to 60 in December

Hoy's Whitefish (Argyrosomus Hoyi) — Usually below 115 meters Small Cottoid (Triglopsis Thompsoni) — Below 115 meters 15

Possibly intensity of light is responsible for the stratification just mentioned, for it seems these dwellers of the depths come to the surface only at night. Practically no light rays penetrate more than 350 feet below the surface of the water. Owls can see by day, but greatly prefer to hunt at night. The lowly forms of life dwelling beneath the surface of the ground have, like the moles, little use for eyes, while the same is true of the blind fish of Mammoth Cave. Parasitic forms that live within the bodies of animals can seldom stand direct exposure to light for any length of time. The germs of syphilis and 15 SHELFORD, V. E. Animal Communities in Temperate America,

p. 82.

tuberculosis are soon killed by sunlight. If a cockroach is set free it will seek the shadow, while the fly goes to the light. In ponds the snails seek a faint light, being found in the lightest spots on the darkest days.

Activity is determined by structure. The fish swims because it is so constructed that it can't help swimming if it moves. We have no reason to think it prefers swimming to walking. For the same reason, birds fly and man walks or runs. It is evident that this coördination has not been produced by the organisms themselves, consciously at least. In the main, they do what they do because of what they are. In the case of lower types they may be carried by wave, wind or animal agency to uncongenial regions and perish. But the higher forms by their power of motion may place themselves in such situation that harm or death must follow. Complete self-realization even for man himself then depends upon maintaining a condition of adaptation. From this there is no exception unaccompanied by penalty.

"If animals are placed in situations where a number of conditions are equally available, they will almost always be found living or staying most of the time in one of the places. The only reason to be assigned to this unequal or local distribution of the animals is that they are not in physiological equilibrium in all the places." 16 Its habitat then is selected by "trial and error," that is if it does not feel comfortable in one place it seeks another till at length, perhaps, it feels content. Animals of the same species will show different habits in different environments. "Animals living in the same places, and apparently under the same external conditions of existence, nevertheless behave in quite different ways under 16 SHELFORD, V. E. o. c., p. 31.

the influence of the various substances held in solution of the water, as salt, oxygen, carbonic acid, etc. The ova of different and yet very closely related forms can endure a long period of drought, or even require it to enable them to develop. Hence, every change, as for instance, in the composition of the water of a lake or river will not affect the fauna inhabiting it equally and as a whole, but will act on some individuals; some will bear the change without being in any way affected by it, others will die, while others again will survive." 17

We must now consider more directly in what ways man himself is influenced by the physical world and for purposes of discussion we may divide this into five heads: (1) distribution and migration, (2) occupation, (3) direct physical effects, (4) physiological effects, (5) psychical effects.

1. Distribution and Migration.- The common belief is that the human race arose in some one area. Even if this be true, we must recognize that in prehistoric times man had made his way to every part of the earth. Though the density of population turns in part on the stage of culture, it is interesting to note his present distribution.

Approximate Density of World's Population, 1911

The frigid zones, deserts, high mountain ranges,

17 SEMPER, K. Animal Life, p. 176.

swamps, water surfaces are sparsely populated, if at all. Most mountain districts impose hard conditions upon men. In Switzerland the mountain cantons show the smallest population per square mile (Grisons, 38; Uri, 48; Valais, 59); while those on the marginal plains are relatively crowded (Zurich, 705; Geneva, 1,356). In England: Westmoreland (85); in Wales: Radnor (49); and in Scotland, Sutherland (11) again show the hill country, for the average density in Great Britain is 374. In Switzerland only 15 per cent of the land can be cultivated and the same is true of Japan. In Norway 67 per cent of the total area is in bare mountains, lakes, snow fields or bogs, 7.6 per cent in pastures, 2.2 per cent in meadows and 7 per cent in grain fields. The dense populations of earth are found in the lowlands below an elevation of 600 feet: in China, the valleys of India, the valleys of the Po, the lowlands of France, Germany and England where the population is over 385 per square mile. The different states in America show the same contrast. We may compare Rhode Island (508), Massachusetts (418) and New Jersey (327) with Montana (2.6), Idaho (3.9) and Louisiana (36.5). Wherever a heavy population is found in the Piedmont between the lowlands and the mountains it is due to the development of mineral resources. Birmingham, Alabama, is a good illustration of this. Unless forced by his enemies or driven by pressure of population, man is very slow to enter the mountains as places of permanent residence. Hence any group getting located in the mountains is likely to drop out of touch with the development of the balance of the country and become "our contemporaneous ancestors" as the inhabitants of the southern Appalachians have been called.

Man's movements on earth have been directed and controlled by physical conditions. High mountain ranges have offered next to the ocean the greatest barriers. Hence has arisen in human history the importance of passes. Thermopyla renowned in Greek literature, the Brenner Pass through which the Germans made their way to the Po Valley, the Dariel Pass in the Caucasus, the Cumberland Gap, the Mohawk Valley and Truckee Pass, through which the California trail in the 40's led, are but a few of historical importance.

In times of peace the passes are the great trade routes determining the location and prosperity of many settlements. The Brenner Pass was largely responsible for the commercial life of Augsburg, Ratisbon, Nuremberg, Leipzig and, in part, Venice. Through the low Mohawk Valley, 445 feet above sea level, was the line of march to the Great Lakes and the West. The district became densely populated while the neighboring Catskills were nearly empty. Because of this great highway Albany, Troy, Utica, Rochester and Buffalo came into being. Had the St. Lawrence been navigable to the lakes and its mouth free from ice the entire development of North America would have been changed, so much does nature determine man's movements. First the animals for ages making their way through the hills, then uncivilized man in pursuit of game, then the trapper and frontiersman following in their trail; then probably the army officer discovering a pass and then the surveyor and the railroad, all following the road indicated by nature. In dozens of places on earth just this development has ensued. For long ages navigable rivers have been favored highways. Wherever the fall line was encountered at the edge of the hilly Piedmont country there too has man