follow the development of an individual through its different ages, we follow the development of the animate world through the phases of its existence that we call the geological ages.
When an old man feels the weight of years, he realizes indeed that his youth has departed from him; but at what moment did he pass from infancy to youth, and then to maturity and old age? He does not know; the phases of his life have unfolded themselves gradually. Things have gone in the same way for all beings. The world has not to-day the physiognomy it once had, but no one can say in what instant it passed from its Primary condition to its Secondary, and from that to its Tertiary, and then to its Quaternary and present. The change of beings has been slow and gradual.
The development of man—that is, of the individual himself—in which the marvels of the animate world are summarized, presents the following phases: 1. Multiplication of constituent parts—that is, numerous points of ossification appear which will become separate bones. 2. Differentiation of parts. As the parts multiply, they differentiate themselves; thus points of ossification similar in the beginning take on differences as they proceed; one becomes the humerus, another the radius, another the orbital bone, etc. 3. Growth of parts. At the same time that they multiply and differentiate themselves they are growing. 4. Progress of activity. Besides material progress, there is progress of a higher order—from the passive existence within his mother's womb till the individual reaches active life and manifests an energy of his own. 5. Progress of sensibility. Sensibility increases at the same time with activity, and sometimes determines it. 6. Progress of intelligence. Finally, intelligence appears. Last come, it also goes away last with sensibility, and will console the old man in the enfeeblement of his other faculties.
The history of the animate world, considered in the aggregate of geological times, is very similar to the history of a man in his brief life. We may study in succession the multiplication of beings on the surface of the globe; their differentiation; their growth; the progress of activity; the progress of sensibility; and the progress of intelligence.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.
