As a matter of fact, there is no absolute rest for them.

Now physiology teaches us that not a fibre works without some expenditure of energy.

The animal, which can be likened, in no small measure, to our industrial machines, demands, on the one hand, the renovation of its organism, which wears out with movement, and, on the other, the maintenance of the heat transformed into action.

We can compare it with the locomotive-engine.

As the iron horse performs its work, it gradually wears out its pistons, its rods, its wheels, its boiler-tubes, all of which have to be made good from time to time.