The effect of this action of ants on the soil-material is peculiar. The tendency is like that noted by Mr. Darwin in the case of earthworms to bring the finer particles to the surface. I am inclined to think the ants accomplished this part of their work even more effectively than the earth-worms, for the reason that they penetrate more deeply between the stones than their less active associates. Like the earth-worms, but in larger measure, the ants convey considerable amounts of organic matter into the soil. Their winter store of food is deeply buried, and much of it remains unconsumed in the nether earth. There is thus a constant inhumation of vegetable matter beneath the materials which they bring to the surface.
Although the burrowing vertebrates operate most vigorously in the forest-covered regions, they also exercise a certain influence on the open country. The moles which work only here and there in the forest are conspicuous agents in overturning the soil in the grassed regions. Still, as this group is peculiarly limited in its distribution, and rarely penetrates to more than four or five inches below the surface, it exercises a relatively small effect. The field-mice are more potent agents in effecting the character of the soil. Their dwelling-chambers are at a considerable depth below the surface, and in forming them, they bring a certain amount of matter to the open air, moreover the remains of their food, as well as their excrements, are important contributions to the organic matter of the soil. Insects in their larval stage exercise a less effect in the open field than in the forest-covered regions; still, they are not to be left out of account in considering the process of soil-making in such areas. In Europe the rabbit, which has a habit of burrowing to a considerable depth, and in certain districts west of the Mississippi, the prairie-dog, overturn the soil on the areas they occupy with considerable rapidity. Still, as the number of these creatures in any given district is not great, their influence is mostly exercised in a very local way.
The foregoing considerations make it tolerably clear that our ants are, in some districts, by far the most important agents in overturning the soil and in commingling the superficial organic matter with the mineral material of which it is composed. Although on a field of a certain class those which are of a clayey nature, the earth-worms, are probably more efficient soil-makers than the ants, this latter group appears to be, at least in the eastern part of North America, on the whole, by far the most effective in the preparation of the soil for the needs of plants. They do not, it is true, take the soil into their bodies and thus disintegrate it, as the earth-worms do, but they accomplish what is perhaps the more important task of rapidly overturning the soil-material as well within the forests as in the open fields wherever that material is of a sandy nature.