obtained from the carbon dioxid which forms some .04 per cent of the air. Under the influence of the sun, for the process takes place in the light only, plants are able through the green substance in their leaves called chlorophyll to appropriate this carbon and embody it in their tissues. No animals have this power unless we make an exception of a few lowly forms like the fresh water polyps, which carry within their bodies algae containing chlorophyll and thus need not wholly depend on an outside supply of proteids. Animals can directly utilize a few substances like salt in small amounts but their continued existence depends on the "predigested food " prepared by plants. The mere existence of animals then is conditioned on the existence of plants. Even the flesheating animals form no exception for "all flesh is grass no matter how many intervening incarnations there may be.

In time the plants die and begin to decay. That is, their substance is used as food by many lowly forms of life and finally, by the joint efforts of different bacteria, are reduced to the simple forms which are again used as food by plants. This decayed vegetable matter we call humus and few plants prosper unless it is present in the soil. This breaking down process is rather complicated and no one sort of bacteria produces all the changes. The first step involves the decomposition of the proteids into ammonia. This is changed by bacterial action into nitrites, and these in turn by union with bases such as calcium and magnesium into nitrates and made available for plants. The nitrite and the nitrate forming bacteria exist side by side in the soil and work together. There are also other bacteria which are able to break down the nitrates and return the free nitrogen to the air. In a

sense then the bacteria in the soil may compete with the plants for food.

It is evident that these bacteria are performing services of the utmost importance and that the life of the higher forms would be impossible without them. They exist in the soil in incredible numbers, up to several million per gram, the greatest number being immediately below the surface. They are nature's scavengers destroying organic compounds. Some of them will live in ice water but the great mass develop at a temperature of 70°. The nonspore-bearing bacteria will perish in water at a temperature of 140° to 150° but the spore-bearing sorts will survive exposure to dry heat of 250° and boiling for an hour.

The proper decay of humus depends on the presence of lime and certain bacteria which draw phosphoric acid directly from the rocks. "In the presence of carbonate of lime and carbon dioxid the insoluble silicate of potash is gradually turned into carbonate of potash, and also into other compounds of the latter." 1 Bacteria aid also in the decomposition of sulphur and iron.

Organic compounds are exceedingly delicate and every one knows how difficult it is to keep them unchanged. We may dry the fruit or meat to such an extent that bacteria does it no harm, but ordinarily the apple or potato rots, the milk sours in spite of our utmost precautions. These changes may or may not be to our liking. Some of them we welcome, others we deplore. The souring of milk is due to a species of bacteria, but if other varieties are present we may have "blue milk," "ropymilk," "red-milk," or "bitter-milk." By using certain bacteria and yeast we produce the sour milks known as

1 LIPMAN, J. G. Bacteria in Relation to Country Life, p. 293 (most of statements relative to bacteria taken from this book).

"kefir," "kumiss" or "matzoon" which are considered of great value. The ripening of the cream is due to bacteria and the average number per c.c. of ripened cream is about 500,000,000. The change to butter and cheese is due to bacteria. We use the yeast plant in the making of bread. Hay is in part the result of bacterial action as is ensilage. So too bacteria are used in the retting of flax and hemp, the tanning of hides, the fermenting of tobacco, the manufacture of vinegar, wine and beer; in the making of pickled fish, sauerkraut and dill-pickles and in the purification of sewage.

Thus nature shows us a most fascinating cycle. The elements, taken from the earth or the air, are utilized to form the bodies of plants and animals and finally are returned to the earth to be used again. Thus the life that now is depends not merely on the coöperation of other forms of life, but also on the life that was. Paradoxically enough the greater the mass of life there is the greater the possible increase other things being equal.

Some animals also help in preparing the earth for other and higher forms of life. The minute protozoa whose remains constitute our beds of chalk are not to be forgotten. The service of the coral polyp in building the islands on which man later lives is well known. In an acre of soil Darwin showed that there were from 50,000 to 500,000 earthworms, each of which passed through its body some 20 ounces of earth a year. On two square yards studied by Darwin the amount equaled 6.75 pounds and 8.38 pounds or at the rate of 1412 to 18 tons per acre per year. Working their way through the earth they open roads for roots and rain. The soil passing through their bodies is softened, and is cast on the surface at a rate estimated at three inches in fifteen years. They

carry leaves into the soil and thus aid in producing the humus. Thomson states that in Yorubaland on the west coast of Africa it is estimated that every particle of soil is brought to the surface once in twenty-seven years. Said Darwin: "It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures." 2 Ants and other insects perform similar

services.

Another type of dependence must be noted. The digestive tract of all the higher animals is densely populated, from a few hours after birth till death by great numbers of micro-organisms. Says Herter: "There is not the least doubt that in some way — a rather intricate way these normal and dominant flora of the intestine exert a protective action on the whole body. This protective action is complex in operation, but consists at least in part of a preëmptive effect on the intestinal domain, by which other types of bacteria are in large degree excluded. It is impossible to avoid introducing into the digestive tract many bacteria which would prove undesirable permanent tenants, and these are successfully discouraged from gaining a foothold by the motile biological activities of the more permanent and better adapted bacterial guests." If the native flora are destroyed or injured "wild" types may be introduced and this Metchinikoff, one of the most eminent biologists, believed to be a common cause of suffering and premature death. This idea is not universally accepted, and many still believe this natural flora to be neutral in influence rather than helpful. With reference to other types we must recognize

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2 THOMSON, J. A. Darwinism and Human Life, pp. 56-59. 3 HERTER, C. A. Biological Aspects of Human Problems, p. 130.

probably that living within our bodies, by their life and death many poisons are produced which are directly injurious to the body. From this standpoint health depends upon a nice adjustment between the natural defenses of the body and the attacks of the invaders. Some of the evil effects will be considered later.

In the preceding chapter we saw that the presence of life in any area depended upon certain physical conditions. It must now be noted that the presence of any of the higher types of life depends also upon the presence of certain other types. In other words, there is a series of interrelations reaching from the lowest to the highest forms. Wherever conditions are favorable the mass of the lowest forms is almost beyond comprehension. I have seen a small pool in Mississippi containing much decaying matter so densely inhabited that the surface was kept in constant motion with a bubbling as of a small brook. To this aggregation of small plants and animals in water we give the collective name plankton. In Lake Michigan Shelford found 11.5 c.c. of this plankton in each cubic meter of water. He states that if the inhabitants of Chicago drink daily 2,000,000 quarts of unfiltered water they imbibe therewith ten solid quarts of plants and animals. He reports that in some of the European lakes ten times this amount is found. In the Illinois River 71.36 c.c. were found, and in an Indiana lake 684 c.c. The largest amount was present from April to June, the least in December and January. The abundance of plant growth depends upon the amount of all the necessary foodstuffs. If any one is absent growth is stopped. As a rule the older and quieter the body of water the greater the plant life. Within certain limits this is also true of animal life. The 4 SHELFORD, V. E. o. c., p. 67.

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