A History of Science, vol 4, Henry Smith Williams [best books to read in life txt] 📗
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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
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