232 THE AMERICAN JOURNAL OF SOCIOLOGY
1,000 in Iceland. At sea-level the limits are reached near the temperature of 75 C.
So long as the conditions of the localities do not change, the same species continue from year to year, barring the inter- vention of man and of domestic animals. However, one may observe some spontaneous phenomena of rotation and alteration. Thus, some species, at first abundant and exclusive, become rare and give place to others. For example, in a prairie the Gramineae give place to the Leguminosae ; forests of oak or of beech replace each other without the intervention of man, etc.
Plants also naturally form certain classes. These individual aggregates of the same species are determined, on the one hand, by the structure of the species itself; on the other, by the conditions of each station. Some plants are in continual war with their neighbors on account of their roots, their height, their foliage, or their excessive propagation. Sometimes, if the battle is permanent, the co-operation is permanent also. The tall forests as well as the feeble Gramineae conserve and develop themselves especially as masses and not as individuals. Further- more, even isolated plants, in spite of their excellence of indi- vidual constitution, have their existence bound up with the fixity of their habitat. The social solidarity has for indestructible foundation the solidarity of everything that lives in the physical environment.
The distribution of cultivated plants obeys the same struc- tural laws. They are domestic species, and each of them has its frontier. The cereals, properly speaking, fulfil their functions only on condition of being annuals ; otherwise they would yield no harvest. But they are annuals only if cultivated outside of the tropical zone, unless in regions quite elevated, so that the cold may cause the stubble to die every year, or unless they become perennial like grass and propagate by sprouts without producing either grain or clusters. This example shows once again in a striking manner the constant static relation which exists between all organic structure, its environment, and its dynamic function.
Barley, oats, and potatoes cannot be cultivated in Europe
