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Page:Popular Science Monthly Volume 32.djvu/502

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By Professor N. S. SHALER.

THE admirable studies of Mr. Darwin on the influence of earth-worms upon the soil has made it clear that these animals exercise a most important effect in its preparation for the use of plants. Mr. Darwin's luminous essay has served to call attention to the effect of organic life on the development of the soil-coating. In the following pages I propose to submit the results of some studies of a general nature, which serve to show that a number of other animals have a considerable influence on the preparation of soils.

Our soils, as is well known, depend upon a variety of actions which serve to break up the rocky matter of the earth, and to commingle that matter with organic materials more rapidly than the erosive agents can remove the detritus from the point at or near which it decays. For the formation of the soil two actions, at least, are essential. First, the bed-rock must be broken into fragments sufficiently separated from each other to permit the passage of roots between them; second, the rock fragments must be still further comminuted and commingled with organic waste to make the combination of organic and inorganic matter on which the utility of the soil absolutely depends. Although the earth-worms are undoubtedly very important agents in overturning and breaking up of soil, it appears to me that they are most effective in the tilled fields or in the natural and artificial grass-lands. So far as I have been able to observe, these creatures are rarely found in our ordinary forests where a thick layer of leaf-mold, commingled with branches, lies upon the earth. The character of this deposit is such that the creatures are not competent to make their way through it, and they therefore in the main avoid such situations. Moreover, wherever the soil is of a very sandy nature, earth-worms are scantily found if they are present at all. These worms are practically limited to the soils of a somewhat clayey character, which have no coating of decayed vegetation upon them.

As the greater portion of the existing soil has been produced in forest regions, I shall first examine the action of various animals upon the soils of wooded countries. The mammals are, of all our vertebrates, the most effective in their action upon the soil of forests. Twenty species or more of our American mammals are burrowers in the forest-bed. They either make their habitations beneath the ground, or resort to it in the pursuit of food. Of these, our burrowing rodents are perhaps the most effective, but a large number of other small mammals resort to the earth and make considerable excavations. In forming their burrows, or in the pursuit of other burrowing animals, these creatures often penetrate through the whole or greater portion of the soil-covering. The material which is withdrawn from the burrow is accumulated