2S 
THE FIRST GREAT DIVISION. 
downwards. The Swan, and some other aquatic bird?, spread their vriiig.s 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 precistdy 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 dfgrcc when it wishes to descend, and dilates this 
organ when about to ascend. This compressive force is accom]ilisliocl by the lateral 
muscles of the body, which tend to contract the bladder by elongating it. lu 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 are 
capabic of having their air-bags so much dilated by licat, 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 cuablo them to descend 
again. 
When the fish is in equilibrium with the water, and wishes to aflvance, its tail is 
bent in two different directions, resembling tlic letter S, by means of its strong and 
complicated lateral umsclos. It then augments the surface of its tail to the utmo.^t 
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 p-irt of this 
velocity is imparted to the fluid, and the body of the fish is propelled onwartls 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 io extend the tailj 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 arc then laid down upon the body, while 
the caudal fin becomes folded and narrow. Again, the curvature of the tail takes 
place very slowly, wfiiln its extension is violent and sudden. On returning to the 
right line, the fall is bent a second time; but this takes place precisely in the opposite 
direction, and the impulse resulting therefrom h:is an equal obliquity only on the 
opposite 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 docs not appear that the pectoral and ventral fins arc of very mneh use in the 
progression of Fishes, Tliey seem intended to aid in picserving them in a state of 
rest, or in equilibrium with the fluid, and they arc extended whenever it becomes ne- 
cesKury to correct the vacillations of tlie body. They arc 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 fiahos would enable us to describe. 
Fishes williout 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 Hays ('limuj elevate themselves wdth their large pectoral fins, 
which arc very properly termed wings, as these fishes use them in rai.sing themselves, 
precisely in the same manner as the bird elevalos itself in the air by means of its 
wings. Unlike the ether fishes, the Flat Fishes (I^leuronectes) arccmnpellcd to swim 
in an oblique position with the back on the one side and the belly on the other, in 
consequence of tboir eyes being placed on the same side of the head. In swimming, 
they acconliugly strike the water from above downwards. As both the Kays and 
Plcuronectes arc 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 dhvetion 
to the whole of their motions. The tail is struck downwards with great force, which 
elevates them sliglnly, and this force, combined with that of gravity, brings them back 
to the horizontal line after describing a curve. They depart from this lino by a new 
leap, in a manner similai- to the flight of Birds. 
The same means ore employed by the M'^hale and other Cetacea. It must be ob- 
sierved, tliat tlie boffius of these I^Iamnialia arc organized for swimir4ing as perfectly as 
those of Fishes ; but they differ from them in this respect, that the efforts of the tail 
arc made principally iu a vertical direction. The use of the air-bag is supplied by 
the lungs, wliich are compressed or dilatetl at will by the action of llie diaphragm and 
the muscles between the ribs. Serpents, and many of the Invcrtcbrated Aiiiniais 
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 b in a state of rest, it forms two angles with the side of the boat, and 
these may either correspond or be unequal. Ihe boatman moves the oar so as to 
render the anterior angle more obtuse, and the Idudcr one more acute. If the water 
did not resist by ils inertia, tho boat could not change its place; but as its resistance 
obstructs the motion of the oar, the angle iu question widens by the progressive mo- 
tion of the boat. When once tho iin|>ulst* has been given, tho boatman draws back 
his oar or turns its cd^e, that it may not interrupt the motion, and then repeats the 
same operation. 
The above dcscvq-.tion of the mechanical process iu rowing, is directly applicable 
to the animals just named, if we only consider their feet as oars, and their bodies as 
so many boats. The Seals, ISIorscs, and other Amphibia, swiiu the most perfectly of 
a:I the IMammalia; while they resemble the Cetacea and Fishes in the form of tlicir 
bodies more uearly 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, tho hinder servo to urge the 
body onwards, and the anterior to sustain the fore-part of the body, which is tlie 
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 IMammalia, he experiences greater difficulty in sup- 
porting his head when swimming than most of tiiesc animak, and he alone, of all 
Mammalia, seems incapable of swimming naturally, and without repeated trials. 
ANALOGIKS OF THE VEUTEBllATED ANIMALS. 
Idenihy of their Construction — Natural Scale of ^nhnal Orffa7iization 
Laws of Moiistrous Development, 
On stating, for the first time, to a person ignorant of Zoology, that all the Vertc- 
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 Mainmiferou^ 
Qaadi'uped, 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 arc all possessed of the same or nearly corresponding propcrticf, 
he may reply, that this is natural to all bodies possessed equally of life. 
It may be necessary Io explain that, when all the Vertebrated Animals are stated 
to be analogous, it is meant that they arc composed of tho 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 the^c animals are all possessed rigorously of 
the same number of organs, or of parts of im organ; on the contrary, many are alto- 
gether deprived of certain parts which arc 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 Maniraalm, possessed of four limbs. It is 
only asserted, that when we investigate the corresponding organs of these animals, wc 
find them to he 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 attempiug to trace these analogous parts, it 
is convenient to select young animals, or rather tlie unborn embryos, in preference 
to the adult animals, because the identity and similarity of tho original and essential 
outline become gradually disguised by those characters, which afterwards form tho 
distinctions of genera and species. 
The exlmial coverings of the Ycrtebratcd Animals appear at first sight destitute of 
every pretoiigion to analogy. We are struck with the remarkable differences between 
the scales of the Fsh, 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 arc all equally covered w'ith a soft 
and thin membrane, — a simple or naked skin ; and, on tracing the subsequent changes 
which this envelope undergoes, wc arc led to perceive productions analogous 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, cr covered with a shield, these arc 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. Wc sec in all the Vcrlc- 
brated Animals a central column of bones, piled one over the other, called vertebra*; 
in all, certain projections or processes arise from these vertebra?; and among the greater 
number, these projections arc suflicienlly lengthened out in front of tho body to form 
ribs, and to constitute the walls of a cavity destined to contain the lieart and the 
lungs. All tliosc peculiaiitics 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, wc find tliat tho first constituent elements arc 
identical — that the points of ossification are the same in all. This appears still more 
evident in the structure of the hc-ad# 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, iu their permanent .vlult states, we should 
be compelled to admit that their differences were more numerous than their analogies, 
ami they would then appear to differ as much in their details as in their general form 
when taken as a whole. But uponoxainining these several heads at the period when 
they first begin to ossify, they exhibit tho 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 tho cause 
of this remarkable contrast in the results, while the materials aro identical, and whence 
it arises that so great an analogy among the elements finally produces sueli evident 
differences in ihc final structure, and in its progress towards comjdeiion. 'I’he cam*®* 
ho^YCver, is involved in impenetrable obscurity; such is the constitutum of Nature, 
and we arc only left to ascertain the facts, without venturing to speculate upon tbe 
cause. 
Thu essential pieces of tho skull are at first of the same number in all the Verte* 
brated Animals; but they have neither tho same form nor size. A certain pa^*’ 
which iu 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 parts to become abortive. There are some heads in which the vomer and the 
bones of the nose arc as largo os the frontal bone ; yet these bones do not on tin* 
