TH£ FIRST GREAT DIVISION. 



downwards. The Swan, and some other aquatic birds, spread their wings to the wind, 

 ■and use them as sails to diminish the labour of paddling. 



■ The body of the Fish is rendered of the same specific gravity as the water, by means 

 of the air-bag or swimming-bladder. By the assistance of this organ, it can raise or 

 depress its body in the water. When this air-bag is burst, the fish remains always 

 on its back, and is unable to ascend. In an ordinary state, the fish is able to com- 

 press the air-bag precisely in the degree sufficient to enable its 'body to remain in 

 equilibrium with the water, and to retain it in the same horizontal plane. It com- 

 presses the air-bag in a greater degree when it wishes to descend, and dilates this 

 organ when about to ascend. This compressive force is accomplished by the lateral 

 muscles of the body, which tend to contract the bladder by elongating it. In this man- 

 ner, though the extent of its surface remains the same, the capacity can be diminished, 

 since it is further removed from a spherical form ; and it is well known that the sphere 

 possesses the greatest solid content of all bodies of equal surface. Some Fishes aro 

 capable of having their air-bags so much dilated by heat, that when they remain for 

 a- long time on the surface of the water, exposed to the burning heat of a tropical 

 sun, they cannot compress the bag in a degree sufficient to enable them to descend 

 again. 



When the fish is in equilibrium with the water, and wishes to advance, its tail is 

 bent in two difi'erent directions, resembling the letter S, by means of its strong and 

 complicated lateral muscles. It then augments the surface of its tail to the utmost 

 extent, by means of the dorsal, anal, and caudal fins. The tail is then extended with 

 great velocity, the resistance of the fluid serves as a solid substance, a part of this 

 velocity is imparted to the fluid, and the body of the fish is propelled onwards by the 

 remainder of that velocity, diminished of course by the resistance of the fluid before 

 the fish. But this is not great, because the force with which it advances is much 

 less than that employed to extend the tail ; and also, when the tail returns to a right 

 line, the fish presents to the fluid only the thickness of its body, which is by no means 

 ■very considerable. 



It is necessary that the fish should bend its tail again to give a second stroke to 

 the water. This motion, however, is directly contrary to the direction of the power 

 by which the tail was extended, and produces in the fluid an equal resistance in the 

 opposite direction. This resistance would be equally powerful, and would completely 

 counteract the progressive motion of the Fish, if the surface of the body continued the 

 same as before. But the anal and dorsal fins are then laid down upon the body, while 

 the caudal fin becomes folded and narrow. Again, the curvature of the tail takes 

 place very slowly, while its extension is violent and sudden. On returning to the 

 right line, the tail is bent a second time; but this takes place precisely in the opposite 

 direction, and the impulse resulting therefrom has an equal obliquity only on the 

 <xpposite side of that imparted by the first stroke. By this means, the course of the 

 body is rendered straight ; and, by striking the water more on the one side than on 

 the other, the fish is enabled to move to the right or left, and to turn round horizon- 

 tally. 



It does not appear that the pectoral and ventral fins are of very much use in the 

 progi'ession of Fishes. They seem intended to aid in preserving them in a state of 

 rest, or in equilibrium with the fluid, and they are extended whenever it becomes ne- 

 cessary to correct the vacillations of the body. They are used also in the slight turn- 

 ings of their progressive motion, and in preventing themselves from falling on one 

 side in swimming. Perhaps in those tribes of fishes where these fins are unusually 

 large, they have some other uses, which a more accurate acquaintance with the habits 

 of these fishes would enable us to describe. 



Fishes without air-bags experience a much greater difficulty in changing their ele- 

 vation in the water. The greater part remain always at the bottom, unless the dis- 

 position of their body enables them to strike the water from above downwards with 

 great force. The Rays (Raia) elevate themselves with their large pectoral fins, 

 ■which are very properly termed wings, as these fishes use them in raising themselves, 

 precisely in the same manner as the bird elevates itself in the air by means of its 

 wings. Unlike the other fishes, the Flat Fishes (Pleuronectes) are compelled to swim 

 in an oblique position with the back on the one side and the belly on the other, in 

 consequence of their eyes being placed on the same side of the head. In swimming, 

 they accordingly strike the water from above downwards. As both the Rays and 

 Pleuronectes are unable to strike the water conveniently to the right and left, they 

 are compelled to make a succession of leaps in order to impart a horizontal direction 

 to the whole of their motions. The tail is struck downwards with great force, which 

 elevates them slightly, and this force, combined with that of gravity, brings them back 

 to the horizontal line after describing a curve. They depart from this line by a new 

 leap, in a manner similar to the flight of Birds. 



The same means are employed by the Whale and other Cetacea. It must be ob- 

 served, that the bodies of these Mammalia are organized for swimming as perfectly as 

 those of Fishes ; but they difl'er from them in this respect, that the efforts of the tail 

 are made principally in a vertical direction. The use of the air-bag is supplied by 

 the lungs, which are compressed or dilated at will by the action of the diaphragm and 

 the muscles between the ribs. Serpents, and many of the Invertebrated Animals 

 having long bodies and no fins, swim in the same manner as Fishes, by suddenly in- 

 flecting their bodies. 



The Quadrupeds, Aquatic Birds, and Reptiles, swim by means of their feet, which 

 propel them onwards in precisely the same manner as a boat is moved by oars. 



When the oar is in a state of rest, it forms two angles with the side of the boat, and 

 these may cither correspond or be unequal. The boatman moves the oar so as to 

 render the anterior angle more obtuse, and the hinder one more acute. If the water 

 did not resist by its inertia, the boat could not change its place; but as its resistance 

 obstructs the motion of the oar, the angle in question widens by the progressive mo- 

 tion of the boat. When once the impulse has been given, the boatman draws back 

 his oar or turns its edge, that it may not interrupt the motion, and then repeats the 

 same operation. 



The above description of the mechanical process in rowing, is directly applicable 

 to the animals just named, if we only consider their feet as oars, and their bodies as 

 so many boats, Tho Seals, Morses, and other Amphibia, swim the most perfectly of 



all the Mammalia; while they resemble the Cetacea and Fishes in the form of their 

 bodies more nearly than any other animals of the same class. In many Quadrupeds 

 the mechanical power of the feet in paddling is greatly increased by membranes 

 between the toes, as may be observed in the Otter and Beaver, but in general they 

 swim simply by the action of the four limbs. Of these, the hinder serve to urge thfe 

 body onwards, and the anterior to sustain the fore-part of the body, which is the 

 heaviest. 



As the weight of the head in Man is greater in proportion to the size of the body 

 than almost any other of the Mammalia, he experiences greater difficulty in sup- 

 porting his head when swimming than most of J,hese animals, and he alone, of all 

 Mammaha, seems incapable of swimming naturally, and without repeated trials. 



ANALOGIES OT THE VERTEBRATE!) AI^IMtAtS. 



Identity of their Construction — Natural Scale of Animal Organization-^ 

 Laws of Monstrous Development, 



On stating, for the first time, to a person ignorant of Zoology, that all the Verte- 

 brated Animals are analogous to each other, he is apt to reject the assertion as para- 

 doxical. He may ask, what analogy can exist between a Serpent and a Mammiferous 

 Quadruped, or between a Frog and a Bird? If we answer, that they are composed 

 of the same elements, of the same tissues, however different they may externally ap- 

 pear, and that they are all possessed of the same or nearly corresponding properties, 

 he may reply, that this is natural to all bodies possessed equally of hfe. 



It may be necessary to explain that, when all the Vertebrated Animals are stated 

 to be analogous, it is meant that they are composed of the same constituent mate- 

 rials, and that, to a certain point, each similar organ is formed of the same number 

 of pieces. By this it is not meant that these animals are all possessed rigorously of 

 the same number of organs, or of parts of an organ ; on the contrary, many are alto- 

 gether deprived of certain parts which are found well developed in other species. It 

 is evident that the Boas, destitute of limbs, cannot resemble in this respect the 

 Fishes provided with fins, or still less the Mammalia, possessed of four limbs. It is 

 only asserted, that when we investigate the corresponding organs of these animals, we 

 find them to be composed of the same materials, and of the same constituent pieces. 



It is unnecessary at present to allude particularly to the internal organs of animals, 

 such as the muscles, arteries, and viscera, as every one is aware that a certain degree of 

 analogy prevails in their internal parts or entrails. It is only necessary to point out 

 some of the leading organs of the animal body, to be assured that there exist undoubted 

 points of analogy in their structure. In attemping to trace these analogous parts, it 

 is convenient to select young animals, or rather the unborn embryos, in preference 

 to the adult animals, because the identity and similarity of the original and essential 

 outline become gradually disguised by those characters, which afterwards form the 

 distinctions of genera and species. 



The external coverings of the Vertebrated Animals appear at first sight destitute of 

 every pretension to analogy. We are struck mth the remarkable differences between 

 the scales of the Fish, and the feathers of the Bird, or between the shield of the 

 Armadillo, and the glittering and delicate skin of the Eel. Yet, if we examine the 

 first periods of the existence of these animals, they are all equally covered with a soft 

 and thin membrane, — a simple or naked skin ; and, on tracing the subsequent changes 

 which this envelope undergoes, we are led to perceive productions analogouB to that 

 same epidermis or cuticle, which invariably forms the external covering of each 

 animal. Whatever changes the outward garb of the animal may undergo, whether it 

 become hairy, scaly, feathered, or covered with a shield, these are merely subsequent 

 modifications which the epidermis or outer skin undergoes in the course of its deve- 

 lopment. 



In the skeleton, these analogies appear still more evident. We see in all the Verte- 

 brated Animals a central column of bones, piled one over the other, called vertebrae; 

 in all, certain projections or processes arise from these vertebras; and among the greater 

 number, these projections are sufficiently lengthened out in front of the body to form 

 ribs, and to constitute the walls of a cavity destined to contain the heart and the 

 lungs. All these peculiarities appear greatly diversified in the different species of 

 full-grown and perfect animals : but, if we look back to their origin, and investigate 

 at the moment that ossification begins, we find that the first constituent elements are 

 identical — that the points of ossification are the same in all. This appears still more 

 evident in the structure of the heads of animals. It seems difficult to conceive that 

 the heads of the Crocodile and of the Sparrow are composed of precisely the same 

 number of pieces as those of Man, or any other of the Mammalia. If we were only to 

 compare the skulls of these several animals, in their permanent a4ult states, we should 

 be compelled to admit that their differences were more numerous than their analogies, 

 and they would then appear to differ as much in their details as in their general form 

 when taken as a whole. But upon examining these several heads at the period when 

 they first begin to ossify, they exhibit the most perfect similarity in the number of 

 pieces of which they arc thus originally composed. 



It would doubtless form a deeply interesting object of inquiry, to ascertain the cause 

 of this remarkable contrast in the results, while the materials are identical, and whence 

 it arises that so great an analogy among the elements finally produces such evident 

 diiferences in the final structure, and in its progress towards completion. The cause, 

 however, is involved in impenetrable obscurity; such is the constitution of Nature, 

 and we are only left to ascertain the facts, without venturing to speculate upon the 

 cause. 



The essential pieces of the skull are at first of the same number in all the Verte- 

 brated Animals ; but they have neither the same form nor size. A certain part, 

 which in one species is excessively reduced in size, takes an extremely large volume 

 in another; and this part, which in some is so greatly developed, occasions the neigh- 

 bouring pai'ts to become abortive. There are some heads in which the vomer and the 

 bones of the nose are as large as the frontal bone; yet these bones do not on thi» 



