lived, increased in volume, and multiplied by division, because the vital exchanges could, not be efficacious and properly regulated except on condition that the mass bore a well-determined proportion to the surface. If the mass became too great, the surface would become insufficient, the mass increasing in proportion to the cube of the radius, and the surface in proportion only to the square of the same radius.
The little protoplasmic masses created under special conditions of the medium would continue in that medium as long as it remained the same. But this medium becomes modified, because it ceases to be what it was—a fact clearly established by geology and paleontology—and we may presume that the modifications were made slowly and progressively. The little protoplasmic masses would also modify themselves and adapt themselves to the conditions created around them. The medium being changed, the living being would be changed too; but, the medium changed, the conditions that had permitted the direct formation of living matter, spontaneous or heterogeneous variation, would also have vanished. The new medium would then be such that the little living masses already created would continue to live, adapting themselves to it, but that new living masses could not be directly formed in it.
The first little masses born on the whole surface of the globe, unless the conditions were much more uniform than they are now, became the starting-point of successive generations, which, obeying the law of progress that presides over evolution and subjected to the conditions of the medium, acquired successively differentiations, very slow, but progressive, which determined in the homogeneous mass the appearance of granulations, localizations, limited condensations, partitionings, networks, etc., that made of the homogeneous droplet a more or less complicated organism. Such were the very slow advances, not becoming perceptible till after very long periods and through millions of successive generations. The nucleus of the cell, the muscular fiber, the nervous cell, the grain of starch, the fatty globule, the secretory cell, etc., were not formed by Nature at the first stroke. They are probably the result of work performed during millions of years and through milliards of generations. These milliards of generations of living droplets or living cells have therefore been as many little laboratories, in each of which has been elaborated, perfected, and differentiated the muscular fiber or grain of starch. Each of these little laboratories has brought to this work some share of activity, and each has added something to the differentiation.
Some, for example, have begun by producing more specially contractile particles in the homogeneous protoplasm; others have
