CHAPTER II

MUTUAL AID AND THE STRUGGLE FOR

EXISTENCE

We have seen that life exists because of the nature of the elements and that it continues to exist if the environment is favorable. We must now consider the various ways in which the different forms of life affect each other. The reader is warned not to interpret all the phenomena to be mentioned as if they resulted from some conscious purpose in the plants or animals. Most of the service rendered, or the harm done, results solely from the nature of the organisms. Thus plants produce purely mechanical effects of the utmost importance. By forming a dense sod they keep the soil on the hillsides from which otherwise it would quickly be removed by water. Their roots extend into the subsoil, then die and decay, thus making openings for air, water and frost which tend to break up the soil, to expose new particles of plant food and thus make increased vegetation possible.

Plants absorb a considerable part of the carbon dioxid given out by animals. Thus they help to make the air fit for animals to breathe just as in water they perform a similar service. They also take up large quantities of water from the earth which pass into the air through the leaves, the amount increasing as the temperature increases, diminishing as it falls. The Washington Elm at Cambridge, Massachusetts, was studied by Professor Pierce of Harvard. He reported that the tree bore some 7,000,000 leaves having a surface area of 200,000 square feet

or about five acres. It being estimated that one acre of grass furnishes 6,400 quarts of water in twenty-four hours, this tree supplied some 32,000 quarts of water daily. The influence of forests is very large. They reduce the mean temperature of the air, increase the humidity, decrease the violence of the winds, offer protection against very hot winds and regulate to some extent the flow-off of the water. Significant as these mechanical changes are they become as nothing when compared to the chemical changes produced by plants.

The common chemical elements found in plants are carbon, nitrogen, hydrogen and oxygen. Plants get the nitrogen and hydrogen largely from the soil, the carbon and oxygen from the air. Now there is very little nitrogen in the rocks, whereas it forms 80 per cent of the air. How is it, then, that cultivated soil contains from .1 per cent to .2 per cent nitrogen, while rich prairie soils have perhaps 25,000 pounds per acre in the top three or four feet? That the leguminous plants (clover, alfalfa, peas and beans) enriched the soil and increased the yield of other crops was known to the Romans of old. It was not until 1886 that Hellriegel was able to show that this was chiefly due to colonies of bacteria which collected in nodules on the roots and were able to draw the nitrogen directly from the air and make it available for plants. It is estimated that an acre of alfalfa adds nitrogen worth at least $25.00 per year to the soil. Most of this supply of nitrogen in the soil must have been taken from the air by earlier generations of bacteria.

Plant tissues contain much carbon. Forty per cent of the weight of rye straw is carbon. An acre of beech forest consumes almost a ton of carbon yearly. This must be

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

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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 141⁄2 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