Page:Popular Science Monthly Volume 9.djvu/243

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which common sensibility is most frequently called into play. And just as this common sensibility is a crude or general sense of touch, so are the several special senses only more or less highly-refined modes of the same sense-endowment. In the case of special tactile organs, of organs of taste and organs of smell, the several contacts between the animal and the body which impresses it, though differing in their delicacy or refinement, are still immediate; while in the case of the organs of hearing and the organs of vision the contact between the sensitive surfaces and the impressing body is mediate, by the intervention in the one case of vibrations transmitted through water or air, and, in the other, of vibrations from the often far-distant luminous body, through an intermediate and all-pervading ether.

The movements of locomotion, or of parts of the organism which become established in correspondence with these various impressions, slowly increase in number, definiteness, and complexity. Such responsive movements, however, are found, as a general rule, to have the effect of prolonging the action of any influences which previous individual or race experiences have proved to be favorable to the life and well-being of the organism; and, on the other hand, of cutting short or avoiding influences which past individual or race experiences have proved to be contrary to its general well-being. The capture and swallowing of food are ends to which a very large proportion indeed of the definite motions of most of the lower organisms are directed; and this direction of their energies is only a special case to be included under the rule above indicated; just as efforts to escape from predatory neighbors are other, though opposite, instances of the same rule.

In addition to the various modes of impressibility by external influence which we have hitherto been considering, there are certain internal modes of impressibility due to changes in the condition of internal parts of the organism. These are commonly spoken of as divisible into two categories: 1. The impressions derivable from, or in some way attendant upon, the contractions of muscles; and, 2. Impressions emanating from one or other of the various sets of internal organs, such as the alimentary canal and its appendages, the respiratory organs, the genital organs, or other internal parts.

With the first set of impressions we have at present nothing to do. They differ altogether from others, whether of external or internal origin, by the fact that they follow or accompany movements whose intensity they are supposed to measure, and do not themselves lead to movements. Granting that such impressions may have a real existence, it is obvious we can know nothing about them among invertebrate animals, if they have only a subjective existence, and do not cause an efflux of molecular movements along outgoing nerve-fibres.

The second category of internal impressions—those emanating