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paration into æsophagus, stomach and intestine. The latter, moreover, is not as in the former attached to a liver. Its posterior extremity certainly opens as in them above the generative organs. But their proper tongue is not situated below but above the entrance of the pharynx. When, however, we speak of development from a common primitive form, and the combination of one form with another, it is not to be understood that these developments and combinations follow the same rules as pairing and generation in the presently existing nature.
3. No conclusion can be drawn from the absence of an organ, regarding its purpose, nor vice versa. An example of this is furnished by the auditory ossicula of fish, discovered by Weber, which are evidently parts of the vertebral column, al. though attached to the organ of hearing, which in other animals is quite unconnected with the column. Another instance is in the mandibules of the crustacea and insects. These organs have the same function as the maxillæ of the vertebrata. But they do not like the latter move in a plane parallel to the axis of the body, but in one at right angles to this axis. In many crustacea, moreover, they perform the function of legs. On the contrary, the organs of these two classes which move in a plane similar to that of the jaws of the higher animals, viz. the upper and under lips, have a totally different function.
To return to our proper subject, the distribution of living existences ; according to our principles of considering life and the intellectual faculties as one and the same, we may be allowed to consider the organs through the medium of which the activity of the living principle passes to the other bodies in the animal kingdom, as those with whose perfection the structure of the other parts must stand in connection. And the more intimate the connection the higher the gradation of life. These organs are the brain, spinal cord, and nerves.
The presence or absence of these organs, as characterising the existence or non-existence of voluntary motion, has always been considered as the basis of the two grand divisions of living nature into animals and plants. No nerves, it is true, can be detected in the simpler animals; but as they have been discovered in some to which they were formerly refused, and as these species are intimately connected with those in which they have
not yet been discovered, it is not improbable that they are an attribute of all animals, even the lowest. It is quite otherwise with plants. Their tissues are so transparent, that if a nervous matter at all analogous to that of the nervous animals existed, it would long ago have been observed.
Here is, therefore, a negative character of plants; and in their classification some other system than the nervous must be made the basis of division. Their positive character consists in the composition of their whole internal substance of solid cells and unbranched vessels. Their cells are separated by intervals, and are connected together in a reticulated form. These intervals have no proper coats.
Globules and not cells are the elementary parts of the animal tissues, and their vessels are always branched. When we approach the confines of the animal and vegetable kingdoms-whenever the vegetable tissues are destitute of solid cells and of vessels, it is always difficult to determine to which of the two kingdoms they ought to be referred, from their want of the distinctive characters of the animal and vegetable tissues.
It might be supposed that the embryo, with its envelopes, was the miniature expression,—the microcosm of the perfectly developed animal, and that the complexity of each were directly proportioned to each other, so that the degree of affinity between organic existences might be concluded from the similarity of their ova and embryos. The appreciation of these resemblances has great or even insurmountable difficulties in animals, but is much easier in plants. The characters deduced from the formation of the embryo and its envelopes, have been found the fittest for the natural arrangement of plants. They are the bases of the Jussieuan System, which I must suppose to be well known to all.
Rudolphi was the first who attempted to arrange animals according to the structure of their nervous system*. He divided them into those with, and those without, a visible nervous system ; but such a division is of no value for biology. Our author has only given the most general distinctions of the classes provided with a nervous system. I have also occupied myself much with this subject; but my contributions are still merely fragments; but I doubt not that a very splendid superstructure
Beiträge zur Anthropologie und Allgemeinen Naturgeschichte.
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 @sophagus. 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,
In the first class from
1: 85 to 1 : 6.5
The relation of the greatest breadth of the cord to the greatest breadth of the brain
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 olfactory, and from the latter the optic nerves. The posterior hemispheres niay 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
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
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