238 THE AMERICAN JOURNAL OF SOCIOLOGY
the same neighborhood, although there is no obstacle or physi- cal barrier to prevent. Certain species common to North America are found under the same latitude in South America. On the contrary, the animals of two countries relatively con- nected, those of the western slope of the Cordilleras and those of Brazil, differ specifically.
The influence of climate and of habitat is generally limited to a more or less complete development of certain members and to a diversity of colors. In other respects this influence oper- ates differently according to the kind or species, each having a power of conservation of type more or less pronounced. But there are for certain determined types impassable barriers, and within the limits assigned by nature this type submits to slow modifications, so that one is not able to determine whether these varieties have come out of the same stock or are products different from each other. 1
To summarize, the exterior geographical, climatic, geological, and elementary environment, in connection with the internal structure of animal species, determines and defines their habitat. The latter extends itself the more easily as the species is the more mobile or becomes more mobile by natural selection and progressive adaptation to the environment. Therefore the fundamental character of the population of the great empires of fauna and the uniformity of structure are disguised only in their exterior appearance. For the human species, which is the most scattered animal species of all, the essential uniformity is altered only by some very limited and, above all, superficial variations which do not destroy the common type.
Let us pass now from these static laws of the distribution of fauna in general, to the observation of the limits of some one of the particular kingdoms. The crustaceans have their peculiar system of distribution. The distribution of their habitats is principally conditioned upon the temperature of water. Accord- ing to Dana, they occupy five principal regions : first, the west- ern region, embracing the American shores of the Atlantic and Pacific Oceans; second, the European region, from Cape Horn
'A. MAURY, La terre et Vhomme.
