disease; dry localities favorable; moist localities are unfavorable.' 13 Malaria has always haunted the lowlands, whereas the mountains have been favored for health resorts. A similar contrast obtains as between wet and dry seasons. With reference to increased rainfall Osborn observes: "(1) it may diminish the supply of harder grasses to which certain quadrupeds have become thoroughly adapted; (2) it may at the same time produce new poisonous or deleterious plants; (3) it may be the means of introducing new insects or other pests, and new insect barriers; (4) it may be the means of introducing new protozoan diseases and new carriers of disease; (5) it may be the means of erecting new forest barriers to migration, or new forest migration tracts for certain carnivora, such as the bears." 14

There are regions so arid that the native animals never seem to drink water but get their supply from the juices of their food, but the higher forms cannot exist under such conditions. The farmer waters his stock at frequent intervals, and cattle in semiarid regions make almost daily trips of considerable distance to get water, leaving the calves exposed to the danger of attack by wolves. Wild horses go for two or three days without drinking and sheep will go for a week or more if necessary. The power of the camel is well known. Yet the water may carry the germs of disease and be deadly rather than life-giving. We can but wonder if the great plagues of history have occurred in wet seasons. The disappearance of the vast herds of horses that inhabited America long before the advent of man is most easily explained on the basis of some disease. It is interesting to note that fossil flies similar to the tsetse flies which carry the sleeping-sickness in Africa to-day have been discovered in America.

Influence of Snow

In as much as a considerable part of the moisture in the colder countries falls as snow, somewhat different results

may be seen. The snow may form a blanket over the earth and protect the grass and bulbs underneath. Wheat suffers much more in "open" winters than in winters with much snow. Animals are often protected by snowbanks. Snow houses are not undervalued by the Eskimo. On the other hand the radiation from the snow lowers the temperature in winter and retards its rise in spring. Hence, April is colder than November. The snow may reflect the rays of the sun and blister the face of man or cause the dreaded snow blindness.

Snow may bury the food supply and cause animals to attack trees or eat the poisonous plants passed by under normal conditions. Horses will scrape away three feet of snow to get at grass, while the cattle perish. The blizzard may bring death to thousands of sheep. The deer will be more exposed to the attacks of wolves if the snow is deep. Heavy snow as well as increased rain may promote the growth of poisonous plants in the country over which stock ranges. The stockmen in Montana have long recognized the danger due to larkspurs, water hemlock, loco weeds, and poison camas. It follows, as a matter of course, that a reduction in the amount of moisture may be beneficial. Malaria disappeared in the Middle West with the draining of the marshes.

Influence of Temperature

The distribution of life is determined by temperature quite as much as by moisture. This statement refers to the temperature during the period of reproduction and infancy rather than to the mean annual temperature. The northward spread of organisms adapted to the tropics 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 thus states it: "Temperature by controlling reproduction pre-determines 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." 16

15

It is customary to speak of the temperature under which any form of life thrives best as the optimum; the lowest at which reproduction takes place as the minimum; the highest as the maximum. There are bacteria which will stand for a time the exceedingly low temperature of liquid air without being killed, while some spores will survive dry heat of 140° C., though they are killed by exposure for thirty minutes to moist heat of 120° C. A few varieties thrive best near the freezing point and a few others whose optimum is between 120° to 140° F. The majority do best at 75° to 85° F. It is for this reason that the lowest forms are so abundant in the warmer parts of the earth.

A survey of conditions in North America, north of southern Mexico, made by the Biological Survey of our government, reveals seven transcontinental life zones "each characterized by particular associations of plants and animals." The Boreal zone has a yearly temperature of less than 10,000° and its hottest period is below 64.4° while the Tropical zone has a yearly temperature above 26,000° and its hottest period is over 78.8°.

It is doubtful if there is a spot on earth where some form of life may not be found, but the distribution is very uneven. Contrast the profusion of plant life in the tropics where growth is possible throughout the year with the frigid regions with their short summers and short growing seasons. Compare the dense tropical forests with their long vines with the little dwarf willows and aspens of the far north. The temperatures are quite as great in the United States as in the tropics during the summer and we may grow many tropical plants in our yards but few of them will produce seed, let alone survive our winters. There are no very small mammals in very cold lands. Too much of their food would

have to be used to maintain bodily temperature. We do not get our fur-bearing animals in the tropics nor our alligator skins from Canada. For reasons not wholly understood the gaudily colored birds like the toucans of the tropics are contrasted with the gray and white grouse and geese of cold lands.

Plants must produce flowers and seed ere the period of rest appears, which means the dry times in the tropics and the winters elsewhere. Since plants and seed mean food to birds and animals their habits must be adjusted to varying conditions. It is not temperature, as such, which causes the great semiannual migration of birds. Many species that winter in the South could remain in the North if food were abundant. Meadow larks, robins, and others sometimes winter in northern states. The winter of 1925-1926 was light on the Pacific coast and thousands of geese and ducks remained in Alaska. It will be noted that the insect-eating birds are the first to start south in the fall, followed by the seed-eaters, and that the order is reversed in the spring migration. The great horned owl, a permanent resident in the North, nests in February while the quail nests in August or later as does the mourning dove at times. The young of wild animals seem to be born at the time of the year when their chances for survival are the best, and temperature, directly or indirectly seems to be the most important factor.

A similar distribution is found in the ocean. The sharks, barracudas, and angel fish characterize the tropics as much as cod, haddock, and halibut do the cold waters. The body temperature of the cod is about 37°. They refuse to remain in water which falls to 32°. In 1882, a section of the Atlantic, 170 miles in length and 25 in width, was covered with dead tile fish, the number being estimated at 1,400,000,000. They live at the edge of the Gulf Stream at a depth of 600 feet. In this year the stream appears to have shifted its course and the fish perished. In 1892 the stream resumed

its old course and the tile fish began to increase and were present in large numbers by 1915. Although there are fossil corals found in Iceland, all living corals are in warm waters, another proof of change of climate in earlier days. Certain fish are always found near the surface, others at lower depths. In Lake Michigan we find the following stratification:

It is possible that the intensity of the light may be responsible for some of the stratification just mentioned, for the denizens of the deeps seem to come to the surface at night only. Bizarre, indeed, are the forms of fish the dredges bring from the depths of the ocean. Practically no rays of light penetrate more than 350 feet below the surface of the water. Be this as it may, light has much to do with the distribution of life. 17 Many lowly forms like the moles living beneath the surface of the ground are blind, as are the fish in Mammoth Cave. Many of the bacteria, especially those which live as parasites in the bodies of animals are quickly killed by exposure to the sunlight. Few germs survive in the bright sunlight of arid regions. Animals vary in their reactions to light. Owls can see by day but prefer to hunt by night. In ponds the snails seek a faint light, being found in the lightest spots on the darkest days. If a cockroach is set free it will seek the shadow, whereas a fly will seek the light. It is in relation to plant growth that light shows the most marked effects, for the formation of chlorophyll takes place in the light only. The contrast in color of the potato sprout