230 THE AMERICAN JOURNAL OF SOCIOLOGY
The commonness or rarity of a species in a country is due to the constitution of each species and to exterior influences. Species are scarcely ever common in a country in proximity to their geographic limit. The individuals are more congested toward the center of the habitat. In the present human divisions the frontier districts are more thinly populated.
Calculation has shown that the average area of species is all the smaller as the class to which they belong has a more perfect organization. This consideration has its importance for sociology. The perfection of the organization is one of the principal factors in the extension of human societies. According to the above law, the cryptogams have the most extensive area; annual plants have a more extensive area than bi-annual ; next come perennial plants; then shrubs and bushes; and, lastly, the most restricted area is that of trees. The average area of planerogamic plants appears to be the greater as the duration of their life is shorter.
Among the secondary conditions it is necessary to reckon the consistence of the soil, the degree of humidity, the abundance of light, etc. There result some stations equally well defined rocks, sand, marsh, and forests. Rarely can the same species live in two of the stations, at least in the same climate.
As to the tertiary conditions, they result from special and numerous modifications which react upon the precedent stations. Thus forests may have deciduous or persistent leaves. The more a station is habitually cold or humid, the more considerable is the proportion of the common species. These causes, in reality, become the predominant factors, which oppose themselves to the variations of the local causes. In this manner is explained the uniform life of northern populations or of those of zones princi- pally maritime, even among the most complex civilizations; with a character, however, less exclusive, as is shown in the industrial development of certain maritime centers.
In dry and warm regions the special and local causes, on the contrary, resume their importance; differences are more con- siderable between the stations, and the vegetation is less uni- form. "It is necessary," says A. Maury, "that in a warm region the same station be very vast, as that of Sahara in Africa and the
