SPARE HALF HOURS

Otago Witness, Issue 1989, 10 April 1890, Page 45

SPARE HALF HOURS

By F. A. Joseph.

DEVELOPMENT OF THE EYE AND

The most highly differentiated organs of the htiman body afe undoubtedly the eyes and ears. The eye With its inimitable contri-» vances for adjusting the focus to pee distinctly objects at different distances, for regulating the amount of light, and the special mechanisms for Correcting BpnerlC, al j and chromatic aberration, far excel the most xngenioris contrivances of the most skilful optician 1 . To watch the development of this most marvellous organ through all it 3 grades in the animal kingdom from the mere sensitive points, enabling an animal to dimly distinguish light from darkness, to the' highly complex organ of vision with which We have been by kind nature endowed, shows us Something like different experimental stages in the perfection of a highly complex organ, were it not that we know that each successive stage but prefigures the more perfect organ in process of development. The simplest organ of vision Consists pt an optic nerve surrounded by pigment cells, ■find Coveted by translucent skin; Such organs, of course, cannot be oalled eyes: they merely serve to distinguish light from darkness, But according to Mr Jourdian there are organs, or rather.difierentiated tissues, of a still simpler nature partaking of the character of eyes. These consist of mere aggregations of pigment cells without any nerve connection whatever. The first outline of the highly complex eye of the higher animals is seen in Certain star fishes, which have, in addition to certain nervea surrounded by pigment cells, depressions in th<? layer of pigment which ,'surrounds the nerve filled with transparent gelatinous matter "projecting with a cornea surface, like the cornea in the higher animals." This organ is not in any sense an eye, as We apply the term to the higher animals, for utar fishes cannot see objects as higher animals can { the convex gelatinous development is merely fdr the ptirpose oi concentrating light, yet in this we see the first outline of the more highly differentiated organ. In order to have a true picture forming eye, we have merely to adjust the naked surface of the optic nerve at the proper distance froifl the light concentrating mechanism. Taking the great division of the animal kingdom classed as articulata or jointed animals, we may start with an optic nerve, simply coated with pigment, and destitute of a lens or any other optical contrivance. In insects, however, the eyes are highly complex, the numerous facets with which their complex eyes are furnished are found to be true lenses, focussing the images upon cones in connection with curiously modified nervous filaments. Bearing upon the development of the eye, Mr A. R. Wallace has remarked : "If a lens has too long or too short a focus, ib may be amended either by an alteration of the curvature or an alteration of density. If the curvature be irregular, and the rays do not converge to a point, then any increased regularity of curvature will be an improvement. So the contraction of the iris and the muscular movements of the eye are neither of them essential to vision, but only improvements which might have been added and perfected at any stage of the construction of the instrument."

Within the range of the highest division of animals — the vertebrates — we find the gradations in the development of the eye very great. Starting with the lancelet, we find that that the eye consists of a " little sac of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other apparatus." In fishes and reptiles, Owen has remarked that " the rage of gradations of dioptric structures is very great." Even in man, according to the high authority of Virchow, the crystalline lens is formed in the embryo by an accumulation of epidermic cells, lying in a sac -like fold of the skin ; and the vitreous part of the eye is formed from embryonic subcutaneous tissue. Writing on this interesting subject, Darwin states: "To arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous, yet absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but I have felt the difficulty far too keenly to be surprised at others hesitating to extend the principle of natural selection to so startling a length. It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument has been perfected by the long continued efforts of the highest human intellects, and we naturally infer that the eye has been formed by asomewhat analogous process. But may not this inference be presumptuous 1 Have we any right to assume that the Creator works by intellectual powers like those of a man ? If we must compare the eye to an optical instrument, we ought, in imagination, to take a thick layer of tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then to consider every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further, we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparentlayers, and carefully preserving each which, under varied circamstances, in any way or in any degree, tends to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million, each to be preserved till a better one is produced, and then the old ones to be all destroyed. In living bodies variation will cause the slight alteration, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years, and during each year or millions of individuals of many kinds, and may we not believe that a living optical instrument might thus be formed as superior to one of glass as the works of the Creator are to tho3e of man ? "

In a manner somewhat analagous we might trace the development of the ear from

the simple arrangement of otolitbs and auditory nerves of a crayfish to the highly complex organ of hearing in men, with its special mechanisms for distinguishing both the intensity and the " pitch " of sound. But from what has been written anyone may gather that the perfected organ has been outlined ; with faint lines at iirst,/beooming clearer and still more clear as the animal rose in the scale of ever-advancing life, till all that was best in all the eyes or ears of all lower forma has been accumulated in the highly differentiated organs of man. If approached in a proper, spirit of reverence the study is fascinating, as it shows us, fallible mortals, hoW the Creator, with Wisdom that never errs; has gradually perfected the higher organs of the ascending animals as they reached a stage qualifying them to make use of better eyes or ears.