and had seen that the honey must be secreted by the petals, and not
deposited from the atmosphere, according to the notion then current.
Kölreuter (1761) had showed that insects may effect the pollination
of flowers. Linnæus (1762) had given the name of _nectary_ to the
honey-gland. He thought that the honey served to moisten the ovary,
though he knew of staminate flowers furnished with nectaries. He
also threw out the alternative conjecture that the honey is food for
insects, which disperse the pollen by their wings. Sprengel improved
upon all his predecessors, and made it clear that transference of
pollen is the main purpose of the honey in flowers. He was put on the
right track by the study of a forget-me-not flower. Here he found the
honey protected from rain by the narrowness of the corolla-tube, whose
entrance was almost closed by internal protuberances. The protuberances
were distinguished by their yellow colour from the sky-blue corolla,
and this conspicuous colouration led Sprengel to infer that insects
might be thereby induced to seek for the store of honey within. He
tested his conjecture by examining other honey-bearing flowers, and
soon collected many instances of spots, lines, folds, and ridges,
which might not only make insects aware of hidden stores of honey, but
guide them to the exact place. Contrivances of the most diverse kinds,
but all tending to invite the visits of insects and utilise them for
the benefit of the plant, rewarded Sprengel's continued inquiries. He
found that night-flowering plants, which could derive no advantage from
coloured patterns, often have large white corollas, easily discerned
in a faint light, and that these flowers give out an odour attractive
to nocturnal insects. He found that the pollen-masses of an orchis
are actually removed by large insects, though here no honey could be
detected in the flower. Sprengel's fertility in probable conjecture
is shown by his explanation of this puzzling case; he suggested that
the orchis is a sham honey-bearer (Scheinsaftblume), which attracts
insects by assuming the conspicuous size and coloration found in most
honied flowers. Darwin suspected, and Herman Müller proved, that
though the spur of the orchis-flower is empty, it yields when pierced
a fluid attractive to bees and other insects. Sprengel discovered too
how insects get imprisoned in the corolla of an Aristolochia, whose
reflexed hairs allow small flies to creep in, but effectually prevent
their escape until they have fertilised the pistils, when the hairs
relax. These are only specimens of a multitude of adaptations which
fill the book.
Sprengel insists upon the study of flowers under natural conditions;
he could never have made out by the examination of plucked flowers
how Nigella is fertilised. Flies with attached pollen-masses, which
he found in spiders' nests, gave him the hint as to the way in which
the fertilisation of orchids is effected. Definite questions must
be put if observation is to be profitable. What is the use of honey
to the plant—of this coloured spot—of these hairs? He notes the
peculiarities of wind-fertilised and insect-fertilised flowers, the
relative abundance of the pollen, the form of the stigma, the presence
or absence of honey, the size, colour, and scent of the corolla. Here
is a pretty illustration from his pages. Pluck a branch of hazel,
aspen, or alder, with unexpanded catkins, and also one from the male
sallow; place them in water, and keep them in a sunny window until
the anthers are ripe. A vigorous puff will then discharge a cloud
of pollen from the _wind-fertilized_ catkins, but none from the
_insect-fertilised_ catkin of the sallow. What Linnæus said about
the flowers of trees appearing before the leaves, in order that the
pollen may more easily reach the stigmas, holds good, Sprengel remarks,
only of wind-fertilised trees. The lime, which is insect-fertilised,
flowers in the height of summer, when all the branches are crowded with
leaves.
Sprengel left it to later biologists to complete his discovery. "That
wonderfully accurate observer, Sprengel," says Darwin,[32] "who first
showed how important a part insects play in the fertilisation of
flowers, called his book _The Secret of Nature Displayed_; yet he
only occasionally saw that the object for which so many curious and
beautiful adaptations have been acquired, was the cross-fertilisation
of distinct plants; and he knew nothing of the benefits which the
offspring thus receive in growth, vigour, and fertility." Not even
Cuvier and the Rise of Palæontology.
If this historical sketch had been prepared within a few years of the
death of Cuvier, it would no doubt have held him up as the greatest
of zoologists and comparative anatomists. Nor would it have been hard
to find reasons for such a verdict. His _Règne Animal_ extended and
corrected the zoological system of Linnæus; his comparative anatomy,
and especially his comparative osteology, were far ampler and more
exact than anything that had been attempted before. It would not have
been forgotten, moreover, that he was the practical founder of the new
science of palæontology.
At a later time, say in the sixties and seventies of the nineteenth
century, when the Origin of Species controversy was in full blast,
any estimate of Cuvier by an evolutionist would have been much less
laudatory. Cuvier had actively opposed that form of evolution which had
been brought forward in his day, and with such power as to close the
discussion for a time. The assailants of the Origin of Species found
his refutation of unity of type and progressive development adaptable
to the new situation, and the reasoning which had pulverised Geoffrey
St. Hilaire was brought out again in order to pulverise Darwin. Then
the supporters of Darwin found it necessary to show that Cuvier was
by no means infallible. This they were able to do without introducing
matter foreign to the main question, for Cuvier's exposition of fixity
of species, of the principles of classification and of the process of
extinction, were entirely opposed to the beliefs not only of Darwin,
but of Lyell and the whole school which stood out for historical
continuity, treated history of every kind as a process of development,
extended almost without limit the duration of life on the earth, and
enforced the obvious but neglected truth that results of any magnitude
whatever may proceed from small causes operating through a sufficient
length of time.
Darwin's main contentions are now accepted by the scientific world, and
Cuvier's hostility to particular forms of evolution has become a mere
historical episode of no lasting importance. Angry disputes concerning
the weight of his authority are at an end; he is not to be blamed
because thirty years after his death he was set up as judge of a cause
which he had not heard. We are now ready to make fair allowance for
the time in which his lot was cast—an age when geology, embryology,
palæontology, and distribution were mere infants, some of them hardly
yet born. We can also admit without reserve the incompetence of certain
of Cuvier's antagonists, and justify the severity with which he treated
unity of type as stated and defended by Geoffroy St. Hilaire. Now
that the dust of controversy has settled, we are chiefly concerned to
inquire: What of all Cuvier's work has proved to be really permanent?
His zoology and his comparative anatomy have had to be completely
re-cast, partly because of the new light thrown on them by embryology
and the doctrine of descent with modification. His studies of extinct
vertebrates, however, called into existence a new science, the science
of Palæontology,[33] and it is mainly this which gives him a lasting
and honoured place in the history of biology.
At the end of the eighteenth century it had been rather grudgingly
admitted that some few animals were actually extinct. Buffon was able
to quote as indubitable examples the mammoth and the mastodon. Their
occurrence in countries unknown to the ancients, such as Siberia
and North America, disposed of the explanation long clung to by the
learned—viz., that their bones were the remains of elephants which had
been led about by the Roman armies, while their large size and the ease
with which they can be recognised rendered it highly improbable that
they still survived anywhere on the surface of the globe.
It was therefore natural that Cuvier's first study in palæontology
should relate to extinct elephants. He compared and distinguished
several species, showed that they were distinct from the existing
Asiatic and African species, a fact which had escaped the notice of
Pallas, and argued from the well-known case of a Siberian mammoth
preserved in ice and frozen mud with hardly any decomposition that
it must have been overwhelmed by a sudden "revolution of the earth."
Whatever we may think of Cuvier's geology, his comparisons of all known
elephants, recent and fossil, introduced a new standard of exactness
into these inquiries. From this beginning he went on to study all
the extinct vertebrates which he could discover in public or private
collections. By 1821 he had published elaborate and well-illustrated
descriptions of near a hundred extinct animals, an extraordinary output
for one investigator.
The most remarkable of his palæontological discoveries were made at
home, in the lower tertiary rocks which underlie the city of Paris. He
proved that in the valley of the Seine a large population of animals,
all now extinct, had formerly flourished. None of these discoveries
impressed his contemporaries more than the celebrated case of the
fossil opossum. The bones were imbedded in a slab of gypsum, and
were at first imperfectly exposed. The lower jaw, however, exhibited
a peculiarity of marsupial or pouched animals, for its angle had an
inwardly projecting shelf, not found in other quadrupeds. The opossums,
like all marsupial animals, bear on the front of the pelvis two long
bones, which support the pouch. These were as yet concealed, and Cuvier
delayed clearing them until he had summoned friends, some of whom may
have been sceptical about the possibility of reasoning with certainty
from anatomical data. Warning them what to expect, he removed with
a sharp tool the film of stone, and revealed the long and slender
marsupial bones.[34] The ancient existence of marsupials in France was
then a striking and almost incredible fact; increase of knowledge has
not lessened its interest, though it has abated some of the wonder.
The fossil ungulates (hoofed quadrupeds) of the Paris basin taxed
Cuvier's patience and skill to the utmost. In the tiresome work of
piecing together a multitude of imperfect skeletons he set an example
to all future palæontologists. That he drew general conclusions which
we are unable to accept, and failed to draw conclusions which seem
obvious to us, will surprise nobody whose reading has taught him how
unprepared were the biologists of that age to handle great questions
concerning the origin and extinction of races. Cuvier recognised among
the fossils of the Paris quarries the bones of two genera of ungulates
very different from any of recent times. One resembled the rhinoceros,
tapir, and horse in being odd-toed; this he called Palæotherium.
Another had the hind-foot even-toed, like a ruminant, though the
fore-foot, with which he was imperfectly acquainted, showed points of
resemblance to the other group. How cautiously he did his work may be
gathered from the fact that he spent fifteen years upon the collection
of facts before he attempted to restore these extinct forms, though
almost every bone in their bodies had during that time passed through
his hands.
The great interest of these fossil ungulates to the modern biologist is
that they are relatively primitive types of the order. Palæotherium is
not far from the ideal common ancestor of the rhinoceros, tapir, and
horse; Anoplotherium not altogether unlike the ideal common ancestor of
the hippopotamus, the swine, and the ruminants. It has been suspected
that Cuvier was less obstinately devoted to the tenet of fixity of
species than he was willing to admit in public. Whatever his private
leanings may have been, he stood out resolutely for cogent proofs of
transmutation. When it was contended that the Palæothere might have
been the remote ancestor of existing ungulates, he demanded that the
intermediate links should be produced. His demand could not be met till
many years later, though intermediate forms between the Palæothere
and the horse have since been furnished in abundance. Reserve about
far-reaching deductions was surely wise at a time when plausible
speculation was rife, and we ought not to judge Cuvier severely for
having aspired to a rigour unattainable in a natural science, and
certainly not always observed by himself. He hoped to see biology
become as exact as astronomy. The hope may have been chimerical, but
emphasis on this side was not altogether out of place in the generation
of Geoffroy St. Hilaire and Oken.
If the great master who laid the foundations of palæontology could
revisit the scene of his former labours, he would find that many
strange things had happened since the appearance of his _Ossemens
Fossiles_. He would perhaps be stupefied at first to discover how
little is now made of the Revolutions of the Earth, the proofs of
which had seemed to him unimpeachable, while the conjectures about
the development of new races, which in his own day had been almost
negligible, have proved to be anticipatory of fundamental biological
truths. The first shock over, one can imagine the zest with which
he would strive to combine the familiar facts into a body of new
doctrine. The ungulates, recent and fossil, would of course interest
him particularly. He would recognise the gradations of structure which
run through the whole order, branching and crossing in all directions;
gradation in the number of the toes, in the rearing of the body more
and more upon the toe-tips, in the progressive complication of the
teeth. One chain of examples would lead from the shallow, tuberculate
molar of the pig to the molar of the horse or ruminant, deep and
massive, with crescentic enamel-folds; another would illustrate the
gradual development of tusks from ordinary incisors or canines; a
third series would show the steps by which the primitive ungulate
dentition became reduced to the dentition of the elephant, with only
a single pair of incisors, enlarged into tusks several feet long,
with no canines but molars of great weight, complicated by extreme
folding. It would surprise and delight him to compare the almost
insensible steps by which his own Palæothere can be seen to pass into
the modern horse. Then we can imagine how our regenerate Cuvier would
draw nearer and nearer to the common ancestor of the whole group, a
five-toed, plantigrade ungulate, with the full dentition of forty-four
unspecialised teeth, and how readily he would admit that Phenacodus,
both in its structure and its geological horizon, was just the common
ancestor that theory required. The proofs of intermediate stages
between ancient and modern ungulates which he had once called for in
vain, he would now find ready to his hand. It might well seem that the
history of the ungulates, with all its modern expansions, would suffice
to occupy even his unparalleled energy. He would see with delight how
the palæontology which he had been the first to treat as a science
has enlarged the comparative anatomy of which also he was so great
a master. He would cheerfully admit that both yield proofs of that
doctrine of descent with modification which a hundred years ago seemed
to him so questionable.
[Illustration: GEORGES CUVIER.
From an engraved copy of the portrait by Pickersgill.]
Chamisso on the Alternation of Generations in Salpa.
Trembley (see p. 57) had shown that Hydra, though an animal, multiplies
by budding like a plant. He got indications, upon which he did not
altogether rely, that it also propagated by eggs, and ten years later
(1754) this supposition was confirmed by Roesel, who figured the egg,
though he was unable to demonstrate that a young Hydra issues from it;
subsequent inquiry has placed the fact beyond doubt. In 1819 Chamisso
announced that Salpa, a well-known Tunicate which abounds at the
surface of the sea, exhibits a regular alternation of the two modes of
increase, the egg-producing form being succeeded by a budding form,
the budding form by the egg-producing form, and so on indefinitely.
Sars a few years later showed that the common jelly-fish Aurelia also
propagates by eggs and buds alternately. Here the familiar swimming
disks, which are of two sexes, produce eggs from which locomotive larva
issue. The larva at length settles down and takes a Hydra-like form.
It pushes upwards an ascending column, which divides transversely and
forms a pile of slices, each destined to become a free, sexual Aurelia.
The alternation of generations may be regarded as resulting from the
introduction of budding into the early stage of a life-history which
culminates in sexual reproduction, much as if a caterpillar were to
divide repeatedly and form more caterpillars, each of which ultimately
became a moth. The case which has been given as an illustration
actually occurs in nature. A parasitic caterpillar, that of Encyrtus,
divides while still an embryo, so that one egg produces several
moths.[35] Many other cases of alternation have since been found among
animals, and it seems to be the rule among plants.
Alternation of generations may be complicated by association with
transformation, by the omission of stages usual in the class, and by
budding-out from one part instead of from the whole body. In particular
cases the complication becomes so great that biological language breaks
down under it. Such terms as _generation_, _individual_,
_organ_, _larva_, _adult_, cannot always be used
consistently without either being strained or artificially limited.
Baer and the Development of Animals.
The curiosity of the ancient Greeks led them to look for the chick
within the egg, and Aristotle mentions the beating of the heart as a
thing which might be observed in a third-day embryo. After the revival
of science Fabricius of Acquapendente figured the chief stages of
development, from the first visible rudiments to the escape from the
egg-shell. Harvey, the discoverer of the circulation, not only studied
the developing chick, but took advantage of the rare opportunity
of dissecting breeding does from the royal parks. His treatise on
Generation is unfortunately impaired by Aristotelian philosophy, and
some of the theories there set forth gave much trouble to Swammerdam.
The oft-cited maxim "Omne vivum ex ovo" does not occur exactly in this
form in Harvey's writings,[36] nor does it fairly state his own belief.
Those who read his _De Generatione_ will see that his knowledge was
insufficient to justify so wide a generalisation; on this head it is
enough to mention that he was persuaded of the production of insects
without parents from putrefying matter.[37]
Malpighi was the first to apply the microscope to the embryonic chick.
His figures are surprisingly full of interesting detail, and so far
in advance of their age that they long failed to produce their due
effect. On one point Malpighi unconsciously led naturalists astray for
a hundred years or more. On examining a fowl's egg which he supposed to
be unincubated, he discovered within it an early embryo. From this he
concluded that the embryo _pre-exists_ in the egg, like a plant-embryo
in a seed. He mentions one circumstance which makes everything
intelligible. The egg was examined in August, during a time of great
heat, and the Italian summer no doubt started development, like the
hot sand of Aden, in which Chinamen hatch their eggs. Swammerdam too
enforced the same belief in pre-existing germs. From the fact that the
butterfly can be revealed by opening the skin of a full-fed caterpillar
he inferred (quite contrary to the opinion which he expresses
elsewhere) that one animal had formed inside another. This led him to
say that there is no such thing as generation in nature, but merely
the expansion of germs which lie enclosed one within another. By his
theory he explained how Levi could pay tribute to Melchizedek before he
was born, and how the sin of Adam can be laid to the charge of all his
posterity. The belief in the pre-existence of germs was first shaken by
Caspar Wolff (see p. 81), who examined unincubated eggs but found no
germ which could be detected by the histological methods then employed.
Swammerdam's _Biblia Naturæ_ contains useful figures of early and
late tadpoles; in particular, he describes a stage in which the body
is entirely composed of rounded "lumps" or "granules," the _cells_
of modern biology.
Early in the nineteenth century Pander and Baer, both of whom were
pupils of Döllinger, a teacher of extraordinary influence, gave a
new impetus to the study of development. Pander (1817-8) published an
account of the early stages of the chick, illustrated by beautiful
plates by D'Alton. Baer (1828-37) carried the work much further, not
only greatly extending the knowledge of the developing chick, but
discovering the mammalian ovum (1827), and announcing generalisations
which down to 1859 were the most luminous that embryology had ever
furnished; we may call him the founder of comparative embryology.
He shows that development may supply decisive indications of the
zoological position of animals; it teaches, for instance, that insects
are of higher grade than arachnids or crustaceans, and that amphibians
ought not to be united with reptiles. He describes the development
of an animal as a process of differentiation, the general becoming
special, and the homogeneous heterogeneous; differentiation is, he
remarks, the law under which not only animals but solar systems
develop. He maintains that the embryo, though gradually attaining
complexity, makes no transition to a different type—_e.g._, the
vertebrate is never in any stage anything but a vertebrate. All
animals, he believes, are probably at first similar, and take the
form of a hollow sphere (the _gastræa_ of modern embryology). There
are, he says, no new formations in nature; all is conversion. When he
comes to speak of the pharyngeal clefts of mammals and birds, recently
discovered by Rathke, he remarks that their correspondence with the
gill-clefts of fishes is obvious. We wonder what is coming next, but
our curiosity is not gratified by any memorable deduction. Neither
here nor in his miscellanies (_Reden_), published nearly fifty
years later, does he admit that mammals and birds can have descended
from gill-breathing vertebrates. If we are inclined to hint that Baer,
having gone so far, might well have gone a little farther, it is only
fair to recollect that every leader in science is more or less open to
the same reproach.
The Cell Theory.
Any one of the higher animals or plants admits of analysis into
_organs_, each adapted to one or more functions. Bichât (1801) showed
that the body of one of the higher animals is not only a collection
of organs, but also a collection of _tissues_, and the same is true
of the higher plants. Analysis of the organism was carried a step
further when in 1838-9 Schleiden and Schwann announced that all the
higher animals and plants are made up of _cells_, which were at first
supposed to consist in every case of a cell-wall, fluid contents, and
a nucleus.[38] It was soon discovered that the cell-wall is as often
absent as present, and that the cell-contents are not simply fluid; the
nucleus is still believed to be universal. Schwann proved that nails,
feathers, and tooth-enamel, though not obviously cellular, consist of
nothing but cells, and it was afterwards shown that bone, cartilage,
fatty tissue, and fibrous tissue arise by the activity of cells which
disappear from view in the abundance of their formed products. The
individual cells of a complex organism are usually themselves alive;
sometimes, as in ciliated epithelium, they give indications of life
longafter they have been separated from the body. The preponderating
importance of the transparent jelly or protoplasm became clear when
it was recognised that this alone is invariably present, and that
this alone responds to stimuli. The nucleus is believed to be only a
specialised part of the cell-protoplasm.
The cell-theory, like nearly every theory, was neither altogether new
nor in its first form altogether complete. Before 1838 cell-division,
as we should now call it, had been indistinctly seen to be the process
by which the body of one of the higher animals is built up. Leeuwenhoek
and Swammerdam had found a wholly cellular stage in frog-embryos (see
p. 103), while Prévost and Dumas in 1824 had in effect discovered that
the cells of which such embryos consist result from repeated division
of an egg; Mohl in 1835 observed the actual division. Even Schwann,
however, was not acquainted with the important fact that every cell
arises by the division of a pre-existing cell.
Swarm-spores of algæ showed that protoplasm, when unenclosed in a
cell-wall, can move about, direct its course, and change its shape.
Knowledge of this fact did more than rectify the definition of the
cell; it effaced one distinction between plants and animals, and gave a
hint of the resemblance of primitive cells to such simple organisms as
Amœba.