are in greater danger from wolves when the snow is deep. Besides driving the rain clouds the winds produce other effects. They greatly increase the evaporation of water and thus affect the humidity of the air. They cause more rapid conduction of heat from the body and thus increase the sensibility to lower temperatures, while if the temperatures are high their effect is pleasing. Some animals hide when the wind blows. To some extent the winds control the distribution of such animals as moths and butterflies and also of plants. They have marked effects on human beings as will be shown later. Temperature is perhaps the most influential factor in determining the distribution of life. This has varying aspects. Indirectly it may control the habitat because of food supply. Many birds could easily spend winter north of the 40° latitude if the food supply were adequate. Robins, blackbirds, meadow larks, turtle doves and others occasionally remain in mild winters in greatly reduced numbers. The codfish have a temperature of about 37°. They refuse to remain in water that falls to 32°. In 1882 a section of the Atlantic 170 miles in length by 25 in width was covered with dead "tile-fish" (Lopholatilus chamæleonticeps) the total number being estimated at 1,400,000,000. They live at the edge of the Gulf Stream at a depth of about 600 feet. In this year the Gulf Stream seems to have shifted its course with the result above stated. In 1892 it resumed its old course and since that time the fish have increased again, being present in large numbers in 1915. In America, north of southern Mexico, there are seven transcontinental life zones: three boreal, three austral and one tropical; "each characterized by particular associa tions of plants and animals." These belts are determined by the temperature. In the development of plants all temperatures above 42.8° are important. Tropical Lower Austral Boreal-below 26,000° hottest period over 78.8° 18,000 hottest period over 78.8 11,500 hottest period below 78.8 10,000 hottest period below 71.6 10,000 hottest period below 64.4 13 The Biological Survey is endeavoring to trace as accurately as possible the actual boundaries of these areas, and is preparing lists of the native animals and of agricultural products that are adapted to each. A most important phase of temperature is its influence on reproduction. The distribution of animals turns not so much on the mean annual temperature as on the mean temperature during the period of reproduction and infancy. Thus the northward spread of tropical plants and animals is limited by the cold of the north, while the southward extension of colder types is limited by the heat. Merriam has called this the "law of temperature control," and he thus states it: "Temperature by controlling reproduction predetermines the possibilities of distribution; it fixes the limits beyond which species cannot pass; it defines broad, transcontinental barriers within which they cannot exist, be the other conditions never so favorable, because of infertility." 14 Many plants of warmer regions will grow in colder regions but without producing seed. It was noted, long before the cause of the disease was discovered, that yellow fever stopped with the oncoming of frosts in the fall. The whale breathes air, yet cannot live out of water. This 13 MERRIAM, C. H. 14 MERRIAM, C. H. Life Zones and Crop Zones, pp. 54-55. is due to the heat produced by the sun. The fat pro tects the whale in water, but kills it in the air. Thus far in the discussion emphasis has been placed upon the physical factors which control and regulate life. If we change the viewpoint and consider primarily life itself, we will be led to speak of adaptation. Really then control and adaptation are but varying aspects of the same process. There are but two sources of oxygen, air and water, and the first great adaptation is to one of these. The lowest forms of life have little specialization of organs so far as we can judge, but in the higher forms we find either lungs or gills. The whale may live in the ocean, provided it comes to the surface to breathe; the flying fish may dart into the air, but must soon seek the wave. The fish does not decompose the water in breathing, but utilizes only the free oxygen carried by the water. Water absorbs and holds various gases. Brook trout thrive if there are 5 c.c. of oxygen to the liter, while if there are 5 c.c. of carbon dioxid to the liter of water, the fish are driven out. Plants thrive by virtue of the carbon dioxid present in water. In an aquarium there is frequently an excess of nitrogen, which causes suffering to the fish. Moreover, the air or water must be of fairly uniform character if the best results are to follow. Pure water like pure air can hardly exist outside the laboratory. Water contains not merely sand and dirt, but likewise numberless substances held in solution, of which the commonest is salt. Few plants or animals accustomed to fresh water can exist in salt, while few of those in the ocean could survive in Great Salt Lake. If the streams are polluted with acids, coal, dirt and filth, the fish soon leave or perish. The fumes from a smelter may destroy 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 15– 19 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. 99 16 "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.' 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. |