THE ORGAN OF VISION.

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divarication from the archetypal plan. Cuvier has happily compared the examination of the comparative anatomy of an organ, in its gradation from its simplest to its most complex state, to an experiment which consists in removing successive portions of the organ, with a view to determine its most essential and important part. In the animal series we see this experiment performed by the hand of nature, without those disturbances which mechanical violence must inevitably produce. We thus learn from comparative anatomy, that the vestibule is the fundamental part of the organ of hearing; and that the other portions, the semicircular canals, the cochlea, the tympanum and its contents, are so many additions made successively to it, according as the increasing perceptive powers of the animals rendered a more delicate acoustic organ necessary.

Comparative anatomy thus discloses the development and differentiation of all the various organs, from the most simple elementary forms to the most finished and complicated structure, and at the same time reveals the regular order of addition of parts on the road to gradual perfection. We might select, for example, the complex and composite structure of the

Organ of Vision,

which, if regarded only in its nearly perfected state, as in man or birds, would seem to have required separate and independent creation in the body in which it is found. But comparative anatomy, in tracing down the structure of the eye, shows us the gradual reduction of parts and brings us at last to the simplest possible arrangement of cells which could serve for visual purpose.

Thus, lowest in the scale of living beings, among the protozoa, we find simply a collection of pigment cells, without any nervous structure whatever, resting upon sensitive

protoplasm. In star-fishes the pigment cells are depressed and the cavities filled with gelatinous matter to constitute primitive lenses and to effect the first concentration of light. In articulate animals an optic nerve is coated with pigment sometimes arranged in the form of a pupil. In insects are superadded numerous facets, each a distinct lens to converge the rays of light upon the sensitive nerve filaments beneath. In the lowest vertebrate animal, the lancelet, we start with "a little sack of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other apparatus." As we ascend the scale of vertebrata we successively encounter the more or less developed crystalline lens, the vitreous and aqueous bodies (additional lenses), the retina with rods or cones, or both, and, finally, the accessory apparatus of muscles and lids and lashes, glands for the manufacture of lubricating oil and tears, and tubes to convey away superfluous fluids.

What is true in this way of organs in the body is true of the whole body, that is of the position it occupies in the animal scale. We may observe in the study of comparative anatomy, the

Order of Succession

of different animals in a progressive and uninterrupted chain. In the vertebrate animals, for instance, we see this succession maintained from fishes to amphibia, reptiles, birds and mammals, and from the lower to the higher orders of each class.

Comparative anatomy pays its highest tribute to biology in displaying the general unity of structure in the various forms of life. We owe the first definite exposition of this principle to the distinguished French philosopher,

Jean Lamarck.

In the earliest years of the present century Lamarck published in Paris his Zoological Philosophy, which was accord

ing to its title "an exposition of the natural history of animals showing the diversity of their organization and faculties the physical causes which maintain them in life, give rise to the movements they may execute and endow them with sentiment and intelligence." Lamarck first propounded the theory of descent in his Natural History of Invertebrated Animals-1815-1822. In this work he maintained that "the systematic divisions of classes, orders, families, genera and species, as well as their designations, are the arbitrary and artificial productions of man. The kinds or species of organisms are of unequal age, developed one after another and show only a relative and temporary persistence. *** The differences in the conditions of life have a modifying influence on the organization, the general form and the parts of animals, and so has the use and disuse of organs. In the first beginning, only the very simplest and lowest animals and plants came into existence; those of a more complex organization only at a later period. The cause of the earth's development and that of its organic inhabitants, was continuous, not interrupted by violent revolutions. Life is purely a physical phenomenon. All the phenomena of life depend on mechanical, physical and chemical causes, which are inherent in the nature of matter itself." Lamarck is therefore justly regarded as the originator of the theory of evolution or the doctrine of descent. His views excited, however, but little attention at the time of publication. Lamarck had no idea of the struggle for existence entailed by numbers and the survival of the fittest, the key notes in the comprehension of the doctrine of descent; still for having proposed the theory and backed it with proof from comparative anatomy, Lamarck must always be regarded as the pioneer in this great discovery.

Almost about the same time, the doctrine of unity of