A History of Science, vol 4 - Henry Smith Williams (epub e ink reader .txt) 📗

fish the blood, after having passed

through their gills, does not return to the heart as from the

lungs of air-breathing animals, but the pulmonary vein taking the

structure of an artery after having received the blood from the

gills, which there gains a more florid color, distributes it to

the other parts of their bodies. The same structure occurs in the

livers of fish, whence we see in those animals two circulations

independent of the power of the heart—viz., that beginning at

the termination of the veins of the gills and branching through

the muscles, and that which passes through the liver; both which

are carried on by the action of those respective arteries and

veins.”[6]

 

Darwin is here a trifle fanciful in forcing the analogy between

plants and animals. The circulatory system of plants is really

not quite so elaborately comparable to that of fishes as he

supposed. But the all-important idea of the uniformity underlying

the seeming diversity of Nature is here exemplified, as elsewhere

in the writings of Erasmus Darwin; and, more specifically, a

clear grasp of the essentials of the function of respiration is

fully demonstrated.

 

ZOOLOGY AT THE CLOSE OF THE EIGHTEENTH CENTURY

 

Several causes conspired to make exploration all the fashion

during the closing epoch of the eighteenth century. New aid to

the navigator had been furnished by the perfected compass and

quadrant, and by the invention of the chronometer; medical

science had banished scurvy, which hitherto had been a perpetual

menace to the voyager; and, above all, the restless spirit of the

age impelled the venturesome to seek novelty in fields altogether

new. Some started for the pole, others tried for a northeast or

northwest passage to India, yet others sought the great

fictitious antarctic continent told of by tradition. All these of

course failed of their immediate purpose, but they added much to

the world’s store of knowledge and its fund of travellers’ tales.

 

Among all these tales none was more remarkable than those which

told of strange living creatures found in antipodal lands. And

here, as did not happen in every field, the narratives were often

substantiated by the exhibition of specimens that admitted no

question. Many a company of explorers returned more or less laden

with such trophies from the animal and vegetable kingdoms, to the

mingled astonishment, delight, and bewilderment of the closet

naturalists. The followers of Linnaeus in the “golden age of

natural history,” a few decades before, had increased the number

of known species of fishes to about four hundred, of birds to one

thousand, of insects to three thousand, and of plants to ten

thousand. But now these sudden accessions from new territories

doubled the figure for plants, tripled it for fish and birds, and

brought the number of described insects above twenty thousand.

Naturally enough, this wealth of new material was sorely puzzling

to the classifiers. The more discerning began to see that the

artificial system of Linnaeus, wonderful and useful as it had

been, must be advanced upon before the new material could be

satisfactorily disposed of. The way to a more natural system,

based on less arbitrary signs, had been pointed out by Jussieu in

botany, but the zoologists were not prepared to make headway

towards such a system until they should gain a wider

understanding of the organisms with which they had to deal

through comprehensive studies of anatomy. Such studies of

individual forms in their relations to the entire scale of

organic beings were pursued in these last decades of the century,

but though two or three most important generalizations were

achieved (notably Kaspar Wolff’s conception of the cell as the

basis of organic life, and Goethe’s all-important doctrine of

metamorphosis of parts), yet, as a whole, the work of the

anatomists of the period was germinative rather than

fruit-bearing. Bichat’s volumes, telling of the recognition of

the fundamental tissues of the body, did not begin to appear till

the last year of the century. The announcement by Cuvier of the

doctrine of correlation of parts bears the same date, but in

general the studies of this great naturalist, which in due time

were to stamp him as the successor of Linnaeus, were as yet only

fairly begun.

 

V. ANATOMY AND PHYSIOLOGY IN THE NINETEENTH CENTURY

 

CUVIER AND THE CORRELATION OF PARTS

 

We have seen that the focal points of the physiological world

towards the close of the eighteenth century were Italy and

England, but when Spallanzani and Hunter passed away the scene

shifted to France. The time was peculiarly propitious, as the

recent advances in many lines of science had brought fresh data

for the student of animal life which were in need of

classification, and, as several minds capable of such a task were

in the field, it was natural that great generalizations should

have come to be quite the fashion. Thus it was that Cuvier came

forward with a brand-new classification of the animal kingdom,

establishing four great types of being, which he called

vertebrates, mollusks, articulates, and radiates. Lamarck had

shortly before established the broad distinction between animals

with and those without a backbone; Cuvier’s Classification

divided the latter—the invertebrates—into three minor groups.

And this division, familiar ever since to all students of

zoology, has only in very recent years been supplanted, and then

not by revolution, but by a further division, which the elaborate

recent studies of lower forms of life seemed to make desirable.

 

In the course of those studies of comparative anatomy which led

to his new classification, Cuvier’s attention was called

constantly to the peculiar co-ordination of parts in each

individual organism. Thus an animal with sharp talons for

catching living prey—as a member of the cat tribe—has also

sharp teeth, adapted for tearing up the flesh of its victim, and

a particular type of stomach, quite different from that of

herbivorous creatures. This adaptation of all the parts of the

animal to one another extends to the most diverse parts of the

organism, and enables the skilled anatomist, from the observation

of a single typical part, to draw inferences as to the structure

of the entire animal—a fact which was of vast aid to Cuvier in

his studies of paleontology. It did not enable Cuvier, nor does

it enable any one else, to reconstruct fully the extinct animal

from observation of a single bone, as has sometimes been

asserted, but what it really does establish, in the hands of an

expert, is sufficiently astonishing.

 

“While the study of the fossil remains of the greater quadrupeds

is more satisfactory,” he writes, “by the clear results which it

affords, than that of the remains of other animals found in a

fossil state, it is also complicated with greater and more

numerous difficulties. Fossil shells are usually found quite

entire, and retaining all the characters requisite for comparing

them with the specimens contained in collections of natural

history, or represented in the works of naturalists. Even the

skeletons of fishes are found more or less entire, so that the

general forms of their bodies can, for the most part, be

ascertained, and usually, at least, their generic and specific

characters are determinable, as these are mostly drawn from their

solid parts. In quadrupeds, on the contrary, even when their

entire skeletons are found, there is great difficulty in

discovering their distinguishing characters, as these are chiefly

founded upon their hairs and colors and other marks which have

disappeared previous to their incrustation. It is also very rare

to find any fossil skeletons of quadrupeds in any degree

approaching to a complete state, as the strata for the most part

only contain separate bones, scattered confusedly and almost

always broken and reduced to fragments, which are the only means

left to naturalists for ascertaining the species or genera to

which they have belonged.

 

“Fortunately comparative anatomy, when thoroughly understood,

enables us to surmount all these difficulties, as a careful

application of its principles instructs us in the correspondences

and dissimilarities of the forms of organized bodies of different

kinds, by which each may be rigorously ascertained from almost

every fragment of its various parts and organs.

 

“Every organized individual forms an entire system of its own,

all the parts of which naturally correspond, and concur to

produce a certain definite purpose, by reciprocal reaction, or by

combining towards the same end. Hence none of these separate

parts can change their forms without a corresponding change in

the other parts of the same animal, and consequently each of

these parts, taken separately, indicates all the other parts to

which it has belonged. Thus, as I have elsewhere shown, if the

viscera of an animal are so organized as only to be fitted for

the digestion of recent flesh, it is also requisite that the jaws

should be so constructed as to fit them for devouring prey; the

claws must be constructed for seizing and tearing it to pieces;

the teeth for cutting and dividing its flesh; the entire system

of the limbs, or organs of motion, for pursuing and overtaking

it; and the organs of sense for discovering it at a distance.

Nature must also have endowed the brain of the animal with

instincts sufficient for concealing itself and for laying plans

to catch its necessary victims… … … .

 

“To enable the animal to carry off its prey when seized, a

corresponding force is requisite in the muscles which elevate the

head, and this necessarily gives rise to a determinate form of

the vertebrae to which these muscles are attached and of the

occiput into which they are inserted. In order that the teeth of

a carnivorous animal may be able to cut the flesh, they require

to be sharp, more or less so in proportion to the greater or less

quantity of flesh that they have to cut. It is requisite that

their roots should be solid and strong, in proportion to the

quantity and size of the bones which they have to break to

pieces. The whole of these circumstances must necessarily

influence the development and form of all the parts which

contribute to move the jaws… … … .

 

After these observations, it will be easily seen that similar

conclusions may be drawn with respect to the limbs of carnivorous

animals, which require particular conformations to fit them for

rapidity of motion in general; and that similar considerations

must influence the forms and connections of the vertebrae and

other bones constituting the trunk of the body, to fit them for

flexibility and readiness of motion in all directions. The bones

also of the nose, of the orbit, and of the ears require certain

forms and structures to fit them for giving perfection to the

senses of smell, sight, and hearing, so necessary to animals of

prey. In short, the shape and structure of the teeth regulate the

forms of the condyle, of the shoulder-blade, and of the claws, in

the same manner as the equation of a curve regulates all its

other properties; and as in regard to any particular curve all

its properties may be ascertained by assuming each separate

property as the foundation of a particular equation, in the same

manner a claw, a shoulder-blade, a condyle, a leg or arm bone, or

any other bone separately considered, enables us to discover the

description of teeth to which they have belonged; and so also

reciprocally we may determine the forms of the other bones from

the teeth. Thus commencing our investigations by a careful

survey of any one bone by itself, a person who is sufficiently

master of the laws of organic structure may, as it were,

reconstruct the whole animal to which that bone belonged.”[1]

 

We have already pointed out that no one is quite able to perform

the necromantic feat suggested in the last sentence; but the

exaggeration is pardonable in the enthusiast to