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ditions. In proportion as we study, we pass from inorganic to vital phenomena, and especially to social phenomena. We ascertain that the growing complexity of phenomena follows a course parallel to that of their modifiability. We cannot at all modify the astronomic structure. We can modify very slightly the geographic and climatic structure of our globe. Climates, continents, and seas have relatively immovable limits. Some plants and animals cross these limits with difficulty, while man transforms the rivers, seas, and oceans into highways of com- munication. He pierces mountains, traverses deserts, and, what is more, he continually modifies and ameliorates himself. How- ever, let it not be forgotten that all of his variations and all of his progress are limited at each moment of time and in each frac- tion of space by constant static laws ; similarly, everywhere and always, each part of every social structure is necessarily in cor- relation with all the others and in the service of the ensemble. In societies, as among individuals and in nature generally, every movement implies a constant equilibration. " Order " and "prog- ress " are two terms which express two inseparable aspects of the same law the law of constant solidarity of every constituent part of social bodies.

SECTION V. DISTRIBUTION OF ANIMALS.

A relatively constant fixity is the law of geographic, climatic, geologic, and mineral structure. This aspect becomes less pre- dominant in the distribution of the vegetable zones and species. Here already, besides the influences of the external environ- ment, other natural causes intervene, of which selection is the principal one, tending to adapt vegetable organisms to special conditions by differentiating them into varieties and species.

The physio-psychic structure of animals introduces in this static complexus new modifying elements, resulting principally from the superior motility derived from their more or less highly developed muscular and intellectual constitution in connection with their nervous system. Muscles and nerves are organs of a more special and more complete life-relation. So the study of laws which involve the distribution of animals upon the surface of the earth is the natural transition to the search after those