Any student may pursue this ruling idea of "adaptation to a functional end," through a vast range of the Animal kingdom. There are eyes fitted to long distances—almost telescopic;—eyes so contrived as to be absolutely microscopical. Then, as the refraction of water differs from that of air, the optical lenses of fishes become rounded almost like little balls. And, the observer who passes into the tribes of Invertebrata, will acquaint himself with eyes mounted upon footstalks,[203] and eyes multiplied and placed in different situations. Few natural objects are more wonderful than the contrivance of a compound eye. The many hexagonal tubes, which may be reckoned by the thousand, are cemented together on one expanded and swollen nervous disk, reminding us of the thalamus in the great plant order of Compositæ, (Syngenesia),—in the Elecampagne for instance, the Bur Marigold, Thistle, and Centaurea. A compound eye has a range of vision extending over about 180 degrees, (half a circle), and must from its structure be endowed with specializing distinctness. Mind in the Universe, is thus presented to us, as in the New Testament,—wide as the whole arch of heaven, but cognizant of a sparrow or a lily.
A creature with diminished vision—such as the Mole—or the Amblyopsis, is a curiously interesting study in itself;—still more so as an example of adaptation.
In old times, the Mole was accounted blind. Aristotle[204] observed that a structural eye exists, but that a skin is drawn over it, and this skin deprives the animal of sight. His observation has made work for commentators, from Simplicius downwards. Trendelenburg (on the De Anima) confines himself to criticism. Torstrik makes a kind of apology for not excising "quæ loco ἀτοπωτάτῳ de talpâ dicuntur." Cardinal Tolet accepts the observation, and thinks the Mole's eyes thus admirably protected from the bad effects of a sudden access of light, when he rushes violently into appearance overground. Naturalists during many centuries, made the whole history of the mole a piece of guesswork, and no creature except the Sloth or the Earwig has ever been more generally misrepresented. Perhaps our familiar old English "Moldwarp" (West of England "Want"), might have remained a puzzle to this day had not a French courtier[205] fled from the Paris Revolution, and devoted his attention to Moles. The fact that the eye of our Western Mole is not completely closed, may be proved by throwing a living specimen into a pond. But, in the South and East of Europe the "blind mole" does really exist,[206] as has been shown by Erhard and the Prince of Musignano. In more than one species, the skin passes over the eyeball without any loss of hair.
This diminution of eyesight is a case of what has been called "retrogression." Now the Mole is a highly developed Mammal, and his position in the animal kingdom entitles him to the best of eyes. But, they would not suit his habits. The same is true of the Blind-fish of Kentucky (Amblyopsis Spelæus). For such a creature, not the distinct vision of objects,—but a sensation of light,—was the desirable possession,—and the creature has it.[207]
It does not in the least matter, as a question of Fitness, whether this retrograde condition of the eye was brought about by natural laws slowly acting upon the animal frame, or produced in some more rapid way. The fitness is the same; and, as we are at present engaged, not on proving the existence of Mind, but in illustrating the greatness of a confessedly existing mind, these instances of far reaching adaptation are very strongly in point.
Of the cavernous life and habits of the Amblyopsis there is not much to be said; though the idea of a happy existence amidst depths of sepulchral gloom, naturally excites our imagination. But "the little gentleman in black" whose health used to be enthusiastically drunk a century and a half ago, is a perfect study[208] in himself. We are interested by his fairy-like gift of hearing (noted by Shakespeare); his gluttony; his fleetness of foot; his combativeness; and his castle-building! As a civil and military engineer, he far surpasses the beaver, though dwelling in dark places, and with only a dubious pair of eyes in his scheming quick-conceiving head.
Probably, the sense we should all least wish to lose is our eyesight. Its perpetual delight, and its capacity for improvement by training are powerful motives for treasuring its possession. The savage and the microscopist, the artist and the astronomer, all train their faculty of vision; and how differently do these four classes of eyes see!—The difference is, we know, in exact proportion to the intelligence which employs and educates them. And, conversely, how the nobly-governed eye informs and educates the Mind! What a world of hope, then, as well as beauty, seems to die when we conceive the blind man in his dim solitude! Yet the contentment of its sightless inmates, is one of the most salient comforts of every blind asylum. Most likely, their cheerfulness depends on the great use of finger-dexterity, and the exquisite susceptibility of the ear. And these delicate endowments, which make our several senses inlets of happiness, are amongst the most fascinating illustrations of the Universal Mind with which we have to do.
The structure of the ear is far less commonly dwelt upon by most writers, than the structure of the eye. Indeed, its organization seems to less certainty explained, the problem being, of course, to trace the transmission of sound to the auditory nerve. But, as in ancient Egypt, so in modern England, the treatment of disease in special organs has been divided amongst special therapeutists; and the ear does not fail to benefit by being better understood. There is, even now, room for hypothesis in some parts of the process of sonorous transmission,—and beyond that process, science does not pretend to go. Modern views, however, as Dr. Tyndall truly says, "present the phenomena in a connected and intelligible form, and should they be doomed to displacement by a more correct or comprehensive theory, it will assuredly be found that the wonder is not diminished by the substitution of the truth." No one has put the wonder into a more intelligible shape than this well known writer, at the close of his book upon Sound.[209]