Out of the materials of former edifices overthrown, new dwellings are constantly being reconstructed.
The labour of our forerunners never wholly perishes.

The ideas of yesterday prepare for those of to-morrow; they contain them, so to speak, _in potentia_.

Science is in some sort a living organism, which gives birth to an indefinite series of new beings taking the places of the old, and which evolves according to the nature of its environment, adapting itself to external conditions, and healing at every step the wounds which contact with reality may have occasioned.
Sometimes this evolution is rapid, sometimes it is slow enough; but it obeys the ordinary laws.

The wants imposed by its surroundings create certain organs in science.

The problems set to physicists by the engineer who wishes to facilitate transport or to produce better illumination, or by the doctor who seeks to know how such and such a remedy acts, or, again, by the physiologist desirous of understanding the mechanism of the gaseous and liquid exchanges between the cell and the outer medium, cause new chapters in physics to appear, and suggest researches adapted to the necessities of actual life.
The evolution of the different parts of physics does not, however, take place with equal speed, because the circumstances in which they are placed are not equally favourable.