There are in this theory, two things to consider: life itself, and the environment amid which it develops. Life is a fixed phenomenon. It began in a marine milieu, at the very beginnings of the world, and it tends constantly to preserve, through all the transformations of a terrestrial milieu, the original conditions of its appearance. As a consequence, the most highly developed animals, the superior animals, among which man takes first place, are those which have been able to preserve in the interior of their bodies, in the form of blood, a vital milieu almost identical with the original marine milieu,--the environment in which life was born: in fact, the degree of saltness in our blood represents the saltness of the sea at the moment life made its appearance, and, moreover, our internal temperature represents the mean temperature of the globe at the moment our species was born. The terrestrial milieu is unstable. Its heat has constantly diminished. Formerly, in the most remote epochs, the vicinity of the poles, now an ice-covered and inaccessible extent, had a climate hotter than that of the tropics. Life was born amid this tropical environment, at the bottom of an ocean that had a far higher temperature than the Caribbean sea or the sea of Java. Nevertheless the poles grew colder and all the other parts of the world as well. Then animal life found itself faced with this alternative: either to accept the new conditions of the milieu, or to rebel against these conditions,--struggle and maintain internally despite the external temperature, the high temperature of its origin. That is a solemn moment in the drama of the world. What is to happen? If the new conditions are accepted, it spells fatal decline. If they are repulsed, it means a magnificent future development. Almost all animal life submitted: it is today represented by the lowest class of living creatures: the invertebrates. A single representative of the animal world revolted, made a prodigious effort, entered into strife with the hostile milieu and dominated it: the vertebrate. Thus life, in its superior aspects, affirmed itself from the very earliest times as an insurrection.