might be raised, if the principles were applied to the filling up

of all the gaps.

According to my view, there are two great divisions of the animal kingdom. The one consists of those provided with a true spinal cord, inclosed within a shut vertebral column, which is always wanting in the other. In the former, the whole brain is situated above the pharynx, and inclosed in an osseous capsule, the skull. In the other there is either a single cerebriform mass, or several nervous knots united together by cords of nervous substance surrounding the œsophagus. The mass analogous to the brain is sometimes above, sometimes below this canal, and is developed in an osseous or horny capsule, which separates it from the other parts of the head; and its posterior part gives out no prolongation comparable with the spinal cord of the other animals. This division, therefore, coincides with that of Lamarck into Vertebrata and Avertebrata. They might also be called Cranial and Acranial animals. We shall, however, adhere to the old epithets of Lamarck, to avoid overloading biology by the introduction of new names.

In all the vertebrata, the anterior extremity of the spinal cord inclosed in the cranium, possesses the same form that it does in man, and in all undergoes a considerable augmentation both in mass and volume, compared with the rest of the cord. Even from the numerical differences of this proportion, four classes can be distinguished in these animals. I found the limits of the proportional weights of the spinal cord to the brain to be,—

The relation of the greatest breadth of the cord to the great

The first class comprehends the mammalia, including man; the second, birds; the third, reptiles; the fourth, fish.

The following are the most important of the other neurological differences of these classes:

In the mammalia, the portions from which the olfactory and optic nerves take their origin, coalesce with the rest of the cerebrum, no limits being distinguishable between them. In all the other vertebrata, the cerebrum is distinguished into anterior and posterior hemispheres, from the former of which arise the olfac tory, and from the latter the optic nerves. The posterior hemispheres may be either separate or united together in the mesial line. They are separate in birds, united in reptiles and fishes. Moreover, these posterior hemispheres may be smaller or larger than the anterior,-smaller in birds, reptiles, rays, and sharks; larger in other fishes.

The mammalia possess a cerebellum, consisting of a middle portion (the vermiform process) and two hemispheres, and its vertical section displays an arborescent prolongation of the cord (arbor vitæ). In birds the hemispheres of the cerebellum are merely rudimentary; the vermiform process, however, still contains a distinct arbor vitæ. This last is entirely wanting in reptiles and fishes, and the cerebellum consists of a simple sack, formed by a thin layer of the cord.

The mammalia alone possess the pons varolii (or great cerebral ganglion).

In fishes, there are found at the base of the brain, behind the optic tracts, two symmetrical eminences, of such extent that they form the greater part of the base, and do not much yield in size to the posterior hemispheres.

The avertebrata, in the same way, fall under two great divisions, according to differences in the structure of their nervous system. In the one, there is extended along the abdomen, in two rows, or in the mesial line, a series of ganglia united to one another, and to the middle of the under half of the cerebriform ring by nervous filaments, and the upper part of the ring consists of two hemispheres, immediately united together. In the other division, there is no series of ganglia extending in regular order the whole length of the animal, and the two lateral tuberosities of the ring are not immediately united, but by means of

bands of nervous substance. To the first division belong those animals whose bodies consist of articulated rings, comprising the crustacea, insects, and worms; to the second, the molluscous animals and zoophytes.

The distinguishing neurological mark between the crustacea and insects, and the worms, is the inequality in the several portions of the abdominal ganglia. This is obvious in the higher insects at the first glance. Their thoracic ganglia always differ in form and size from the abdominal. The difference is not so marked in the genera nearest to the worms, the Millipes, Scolopendrum, and Julus. But even here the ganglia, from which arise the nerves supplying the organs of generation, are different in size from the others; whereas in the worms, no other distinction is observable between the ganglia than a general decrease in size and relative distance towards the posterior end of the animal. They have, in common with most crustacea and insects, the possession of but one series of ganglia situated in the middle line of the abdomen, and composed of a symmetrical right and left half. A deviation from this structure exists in the phalangiæ which possess ganglia on both sides of the abdomen, but united with one another only by simple filaments.

This first division passes into the second by the cirrhipedes, which, in the articulated posterior part of their bodies, possess a series of ganglia, as in the former animals, but in the upper portion of their cerebral ring, no hemispheres immediately connected together. Among the mollusca, there are families distinguished from all other animals by the want of symmetry of their brain. In those where the cerebral ring is symmetric, it has no central mass. Nothing certain is known of the nervous system of the zoophytes. But from the general radiated structure of this class of animals, it is probable that it is merely a simple cord, surrounding the orifice of the alimentary canal, or an aggregate of ganglia united in the form of a ring, and which give off nerves in a radiated manner.

Much is still wanting to carry out this neurological classification in all its details; and even did we possess an adequate mass of observations, a more uniform division could hardly be proposed. It will, however, be by no means out of place to trace the concordance of the characters deduced from the nervous

system, with the natural characters of the families for the class of mammalia.

The proportion of the spinal cord to the encephalon which we have already found of such value in the primary divisions of the vertebrata, is no less important in their further subdivision. I have found it to correspond with, and to follow, analogous gradations, with the natural characters in the genera and species. Of these dimensions, the most important are the greatest breadths of the two organs, which agrees with their relative weights, and very nearly with the transverse diameters of the cranial cavities, and of the occipital foramen. In the following table, I have brought together several of these relations, and have ranged the species according to their natural affinities. Those marked with an asterisk are taken from Tiedemann's Icones Cerebri Simiarum et quorundam Mammalium Rariorum; the others are from my own observation. Most of these last depend on immediate measurements of the brain and cord. Those marked with a cross, are the relative breadths of the occipital foramen and of the cranial cavity.

The greatest breadth of the spinal cord immediately behind the pons is to the greatest breadth of the cerebrum,

We have here.a double series, an increasing and a decreasing. The first ranges from man to the Virginian Opossum ; the second continues from the Opossum to the Cetacea. The intermediate members of the first series are, first, the apes, then the makis (Lemur), then the plantigrade or digitigrade carnivora, and, lastly, the insectivorous genera of Erinaceus, Talpa, and Vespertilio. In the second series, the links are filled up by the Rodentia, of which the hare forms the passage to the Ruminantia ; to which follows the Pachydermata, which pass through the Phocæ into the Cetacea. Thus the relations of the spinal cord to the brain in these animals, entirely coincide with their other natural affinities. Some writers, it is true, have regarded their affinities differently, making the bat succeed to the makis, and the phoca to the plantigrades. But, in the determination of these affinities, we must not be swayed by a few of the more obvious external marks, but by the whole internal as well as external conformation. Upon such a basis these animals will assume the order that we have given them above. The only analogy of the bats with the makis is in the pectoral mammæ; and the phocæ are intimately unconnected with the cetacea through the genus Manatus.

By this, however, I am far from asserting the mammalia can be ranged in an uninterrupted series, according to the proportions of the brain to the cord. Even in the above list, deviations are found from such an order. This is owing partly to the relative breadths of these two organs not being exactly indicative of their relative masses; and to the circumstance of the number of individuals in which the masses are determined with precision being still too small to justify the formation of an exact series. Only, if these defects were supplied, it might be expected that, in order to preserve the scale of natural affinities, the vacuities of the ascending and descending series should be filled up. It cannot be doubted, for example, that, in the sloths (bradypus), the brain, compared with the cord, is smaller than in the makis and the uppermost of the carnivora. They are, however, immediately connected with the makis, and must be placed in the series between the lemur and ursus. If the proportional breadths of the organs were as 100 to 300, the descending series from man to the opossum would change into two others, one from man