the disposition of the facets, obvious examples of optical adaptation.

In the Asaphus caudatus (see Pl. 45, Figs. 9 and 10.,) each eye contains at least 400 nearly spherical lenses fixed in separate compartments on the surface of the cornea.* The form of the general cornea is peculiarly adapted to the uses of an animal destined to live at the bottom of the water: to look downwards was as much impossible as it was unnecessary to a creature living at the bottom; but for horizontal vision in every direction the contrivance is complete.† The form of each eye is nearly that of the frustum of a cone (see Pl. 45, Figs. 9 and 10.,) incomplete on that side only which is directly opposite to the corresponding side of the other eye, and in which if facets were present, their chief range would be towards each other across the head, where no vision was required. The exterior of each eye, like a circular bastion, ranges nearly round three-fourths of a circle, each commanding so much of the horizon, that where the distinct vision of one eye ceases, that of the other eye begins, so that in the horizontal direction the combined range of both eyes was panoramic.

If we compare this disposition of the eyes with that in the three cognate Crustaceans, by which we have been illustrating the general structure of the Trilobites, we shall find the same mechanism pervading them all, modified by peculiar adaptations to the state and habits of each; thus in the Branchipus (Pl. 45, Fig. 3, b, b',) which moves with rapidity in all directions through the water, and requires universal

* As the Crystalline lens in the eyes of Fishes is spherical, and those in the Eye of Trilobites are nearly so, there seems to be in this form an adaptation to the medium of Water, which would lead us to expect to find a similar form of lens in the compound Eyes of all marine Crustacea, and probably a different form in the compound Eyes of Insects that live in Air.

†The facetted eyes of Bees are disposed most favourably for horizontal vision, and for looking downwards.-Lib. Ent. Knowl. v. xii. p. 130.

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vision, each eye is nearly hemispherical, and placed on a peduncle by which it is projected to the distance requisite to effect this purpose. (See Pl. 45, Fig. 3, b, and b'.)

In the Serolis (Pl. 45, Fig. 6. b'.,) the disposition of the eye, and its range of vision, are similar to those in the Trilobite; but the summit of the eye is less elevated; as the flat back of this animal presents little obstruction to the rays of light from surrounding objects.*

In the Limulus (Pl. 45, Fig. 1.,) where the side eyes (b, b') are sessile, and do not command the space immediately before the head, two other simple eyes (b") are fixed in front, compensating for the want of range in the compound eyes over objects in that direction.†

In the above comparison of the eyes of Trilobites, with those of the Limulus, Serolis, and Branchipus, we have placed side by side, examples of the construction of that most delicate and complex organ the eye, selected from each extreme, and from a midway place in the progressive series of animal creations. We find in Trilobites of the Transition rocks, which were among the most ancient forms of animal life, the same modifications of this organ which are at the present time adapted to similar functions in the living Serolis. The same kind of instrument was also employed in those middle periods of geological chronology when the Secondary strata were deposited at the bottom of a warm sea, inhabited by Limuli, in the regions of Europe which now form the elevated plains of central Germany.

Fig. 1. b', Fig. 3. b'. and Fig. 6. b'. are magnified representations of the eyes to which these figures are respectively adjacent. Figs. 10. and 11. are differently magnified forms of the eye of Asaphus caudatus, which in Fig. 9. is represented of its natural size. A few of these lenses are semitransparent; they are still set in their original rims, or frame-work of the cornea, the whole being converted into calcareous spar.

†These eyes are placed so close together, that, having been mistaken for a single eye, they caused the name of Monoculus Polyphemus to be applied to this animal by Linnæus.

The results arising from these facts are not confined to animal Physiology; they give information also regarding the condition of the ancient Sea and ancient Atmosphere, and the relations of both these media to Light, at that remote period when the earliest marine animals were furnished with instruments of vision, in which the minute optical adaptations were the same that impart the perception of light to Crustaceans now living at the bottom of the

sea.

With respect to the waters wherein the Trilobites maintained their existence throughout the entire period of the Transition formation, we conclude that they could not have been that imaginary turbid and compound Chaotic fluid, from the precipitates of which some Geologists have subposed the materials of the surface of the earth to be derived; because the structure of the eyes of these animals is such, that any kind of fluid in which they could have been efficient at the bottom, must have been pure and transparent enough to allow the passage of light to organs of vision, the nature of which is so fully disclosed by the state of perfection in which they are preserved.

With regard to the Atmosphere also we infer, that had it differed materially from its actual condition, it might have so far affected the rays of Light, that a corresponding difference from the eyes of existing Crustaceans would have been found in the organs on which the impressions of such rays were then received.

Regarding Light itself also, we learn from the resemblance of these most ancient organizations to existing eyes, that the mutual relations of Light to the Eye, and of the Eye to Light, were the same at the time when Crustaceans endowed with the faculty of vision were first placed at the bottom of the primeval seas, as at the present moment.

Thus we find among the earliest organic remains, an Optical instrument of most curious construction, adapted to produce vision of a peculiar kind, in the then existing repre

sentatives of one great Class in the Articulated division of the Animal Kingdom. We do not find this instrument passing onwards, as it were, through a series of experimental changes, from more simple into more complex forms; it was created at the very first, in the fulness of perfect adaptation to the uses and condition of the class of creatures, to which this kind of eye has ever been, and is still appropriate.

If we should discover a microscope, or telescope, in the hand of an Egyptian Mummy, or beneath the ruins of Herculaneum, it would be impossible to deny that a knowledge of the principles of Optics existed in the mind by which such an instrument had been contrived. The same inference follows, but with cumulative force, when we see nearly four hundred microscopic lenses set side by side, in the compound eye of a fossil Trilobite; and the weight of the argument is multiplied a thousand fold, when we look to the infinite variety of adaptations by which similar instruments have been modified, through endless genera and species, from the long-lost Trilobites, of the Transition strata, through the extinct Crustaceans of the Secondary and Tertiary formations, and thence onward throughout existing Crustaceans, and the countless hosts of living Insects.

It appears impossible to resist the conclusions as to Unity of Design in a common Author, which are thus attested by such cumulative evidences of Creative Intelligence and Power; both, as infinitely surpassing the most exalted faculties of the human mind, as the mechanisms of the natural world, when magnified by the highest microscopes, are found to transcend the most perfect productions of hu

man art.

SECTION III.

Third Class of Articulated Animals.

FOSSIL ARACHNIDANS.

UNDER the relations that now subsist between the animal and vegetable kingdoms, the connexion of terrestrial Plants with Insects is so direct and universal, that each species of plant is considered to afford nutriment to three or four species of insects. The General principle which we have traced throughout the Secondary and Tertiary formations, ever operating to maintain on the surface of the earth the greatest possible amount of life, affords a strong antecedent probability that so large a mass of terrestrial vegetables as that which is preserved in the Carboniferous strata of the Transition series, held the same relation, as the basis of nutriment to Insect families of this early date, that modern vegetables do to this most numerous class of existing terrestrial animals.

Still farther, the actual provisions for restraining this Insect class within due bounds, by the controlling agency of the carnivorous Arachnidans would lead us to expect that Spiders and Scorpions were employed in similar service during the successive geological epochs, in which we have evidence of the abundant growth of terrestrial vegetables.

Some recent discoveries confirm the argument from these analogies, by the test of actual observation. The two great families in the higher order of living Arachnidans (Pulmonariæ) are Spiders and Scorpions; and we have evidence to show that fossil remains of both these families exist in strata of very high antiquity.