GENERAL DIFFERENCE OF THE TWO LIVES WITH REGARD TO THE MODE OF ACTION OF THEIR RESPECTIVE ORGANS.
Harmony is to the functions of the organs, what symmetry is to their conformation; it supposes a perfect equality of force and action, between their similar parts, just as symmetry indicates an exact analogy of external form, and internal structure: indeed it is a necessary consequence of symmetry, for two parts essentially alike in structure, cannot much differ in their manner of action. Hence we should be naturally led to the following conclusion, namely, that harmony is the character of the animal, discordance that of the organic functions. But on these points we must be more particular.
I. _Of harmony of action in the animal life._
We have already observed, that the animal life arises from the successive actions of the senses, the nerves, the brain, the locomotive organs, and the voice. We shall now consider what harmony of action is, in each of these great divisions.
The precision of our sensations appears to be the more complete in proportion as there exists a resemblance between the two impressions, of which they are each of them the assemblage. We see inaccurately when one of the eyes is better formed, and stronger than the other; when it conveys to the brain a clearer image than its fellow does. It is to avoid this confusion that we shut one eye, while the action of the other is increased by the application of a lens, for at such time there can be no harmony of action in the two organs; accordingly, we make use of one of them only in order to avoid the discordance of the impression.--What a lens applied to one eye only produces artificially, is exemplified in a natural way by squinting.--We squint, says Buffon, because we turn the weaker eye from the object on which the stronger is fixed; for in so doing we avoid the confusion, which would arise from the perception of two dissimilar images.
We know that many other causes may contribute to the production of this effect, but we cannot doubt the reality of the reason assigned. We know also, that in some animals each eye may act without the assistance of the other, and that two different objects may be transmitted at the same time by the two eyes of certain other animals; but this circumstance, when the action of both the organs is united upon a single object, should by no means prevent a similarity in the two impressions. A single sensation is the consequence of the combination; but in what way can such sensation be formed with accuracy, if the same body at the same time be pictured both in strong and weak colours on the one and the other of the retinæ?