All we can ask is that the means shall be adapted to the end; in other terms, that the myriad of large or small local or central pieces shall be determined, adjusted, and coordinated in view of the final and total effect to which they co-operate nearly or remotely.
But, whether simple or compound, every engine which does any work is subject to one condition; the better it is suited to any distinct purpose the less it is suited to other purposes; as its perfection increases, so does its application become limited .-- Accordingly, if there are two distinct instruments applied to two distinct objects, the more perfect they are, each of its kind, the more do their domains become circumscribed and opposed to each other; as one of them becomes more capable of doing its own work it becomes more incapable of doing the work of the other; finally, neither can take the place of the other, and this is true whatever the instrument may be, mechanical, physiological, or social.
At the very lowest grade of human industry the savage possesses but one tool; with his cutting or pointed bit of stone he kills, breaks, splits, bores, saws, and carves; the instrument suffices, in the main, for all sorts of services.

After this come the lance, the hatchet, the hammer, the punch, the saw, the knife, each adapted to a distinct purpose and less efficacious outside of that purpose: one cannot saw well with a knife, and one cuts badly with a saw.

Later, highly-perfected engines appear, and, wholly special, the sewing-machine and the typewriter: it is impossible to sew with the typewriter or write with the sewing-machine .-- In like manner, when at the lowest round of the organic ladder the animal is simply a shapeless jelly, homogeneous and viscous, all parts of it are equally suited to all functions; the amoebae, indifferently and by all the cells of its body, can walk, seize, swallow, digest, breathe, and circulate all its fluids, expel its waste, and propagate its species.

A little higher up, in fresh-water polyp, the internal sac which digests and the outer skin which serves to envelop it can, if absolutely necessary, change their functions; if you turn the animal inside out like a glove it continues to live; its skin, become internal, fulfills the office of a stomach; its stomach, become external, fulfills the office of an envelope.

But, the higher we ascend, the more do the organs, complicated by the division and subdivision of labor, diverge, each to its own side, and refuse to take each other's place.