as early as the second half of the seventeenth century he unravelled
the tissues of many animals and plants. His work on plant-tissues was
so closely accompanied by the similar researches of an Englishman,
Nehemiah Grew, that it is not easy to assign the priority to either.
Malpighi was the first to demonstrate the capillaries which connect the
arteries with the veins, the first to investigate the glands of the
human body and the sensory papillæ of the skin. At the request of our
Royal Society he drew up an account of the structure and life-history
of the silkworm, which is memorable as the earliest anatomical study of
any insect. Malpighi also applied his microscope to the chick-embryo,
and figured its chief stages. His exposition of the formation of the
heart and vessels of the chick is a marvellous example of the quick
appreciation of novel structures.
If we suppose the _Micrographia_ of Hooke to be greatly enlarged,
so as to become, instead of the passing occupation of a man busied with
a hundred other interests, the main pursuit of a long and laborious
life, we shall get a rough notion of the microscopic revelations
of Leeuwenhoek. His researches were desultory, though not quite
so desultory as Hooke's; he must have often spent months upon an
investigation which Hooke would have dismissed in as many weeks.
Both travelled over the whole realm of nature, and lacked that
concentration which made the work of Swammerdam so productive and so
lasting.
[Illustration: ANTONY VAN LEEUWENHOEK.
From the portrait by Verkolje, prefixed to the _Epistolæ ad Soc. Reg.
Angl._, Leyden, 1719.]
Leeuwenhoek worked with simple lenses, ground and mounted by his own
hands. It was easy to make lenses of high magnifying power, but hard to
correct their optical defects, to bring a sufficiently strong light to
bear upon the object, and to focus the lens. When he wished to send out
his preparations for examination by others, he found it best to fix the
objects in the focus, and to provide each with a separate lens. With
such microscopes he managed to study and figure very minute objects,
such as blood-corpuscles, spermatozoa, and bacteria. The spermatozoa
were brought under his notice by a young Dutch physician named Hamm;
but it was Leeuwenhoek's account of them, and his daring theory
of their physiological _rôle_, which gave them such celebrity. To
Leeuwenhoek we owe the first discovery of the rotifers, the infusoria,
Hydra, the yeast-cell, the bacteria, and the generation of aphids
without male parents.
The tradition of the minute anatomists has never been lost, though
we shall be unable to pursue it in these pages. Lyonet (see p. 61)
even surpassed Swammerdam in the elaborate finish of some of his
insect-dissections.
Early Notions about the Nature of Fossils.
Throughout the sixteenth century naturalists held animated debates
about the shells which are found far from the sea, and even on the
top of high hills. Had they ever formed part of living animals or
not? Such a question could hardly have been seriously discussed among
simple-minded people; but the learned men of the sixteenth century were
rarely simple-minded. They had been trained to argue, and argument
could make it plausible that such shapes as these were generated by
fermentation or by the influence of the stars. So prevalent were these
doctrines that it entitles any early philosopher to the respect of
later generations that he should have taken shells, bones, and teeth to
be evidences of animal life. In this singular roll of honour we find
the names of Cesalpini, Palissy, Scilla, Stenson, Hooke, and Woodward.
In England the struggle between philosophy and common-sense was long
kept up. Dr. Ralph Cudworth of Cambridge taught that there is in nature
a subordinate creative force of limited power and wisdom, to whose
imperfections may be attributed the "errors and bungles" which now and
then mar the work. To this subordinate creative force he gave the name
of "vegetative soul," or "plastic nature." None but Cambridge men, it
would appear, felt the weight of Cudworth's reasoning; but several
of these, and especially John Ray[6] and Martin Lister, defended his
conclusions in published treatises. Lister, in a chapter devoted to
"cochlites," or shell-shaped stones, pointed out that they differ from
true shells in being of larger size, in occurring far from the sea, in
being formed of mere stony substance, and in being often imperfect.
Some naturalists had conjectured that the living animals of the
cochlites still exist at great depths in the sea, but Lister evidently
thought otherwise.
In the eighteenth century the belief that fossils are the remains
of actual animals and plants more and more prevailed, the death and
sealing up of the organisms being generally attributed to Noah's
flood. The occurrence of fossils on high mountains seemed so strong
a confirmation of the Biblical narrative that Voltaire was driven
to invent puerile explanations in order to dispel an inference so
unwelcome to him. By the end of the century most naturalists accepted
the doctrine that the great majority of fossils are the remains of
organisms now extinct—a doctrine which was enforced by the remarkable
discoveries of Cuvier (see p. 93). Nearly at the same time William
Smith established the important truth that almost every fossil marks
with considerable precision a particular stage in the earth's history.
Comparative Anatomy: the Study of Biological Types.
Between 1660 and 1740 the scope of natural history became sensibly
enlarged. System had been hitherto predominant, but the systems had
been partial, treating the vertebrate animals and the flowering plants
with as much detail as the state of knowledge allowed, but almost
ignoring the invertebrates and the cryptogams. System was now studied
more eagerly than ever by such naturalists as Ray and Linnæus, but new
aspects of natural history were considered, new methods practised,
new groups of organisms included. Many remarkable vertebrates were
anatomically examined for the first time. Claude Perrault and his
colleagues of the Académie des Sciences dissected animals which had
died in the royal menagerie, and compared the parts and organs of
one animal with those of another; Duverney compared the paw of the
lion with the human hand; in England Tyson studied the anatomy of the
chimpanzee, porpoise, opossum, and rattlesnake, searching everywhere
for the transitions which he believed to connect all organisms, and
to form "Nature's Clew in this wonderful labyrinth of the Creation."
The new microscopes helped to bring the lower and smaller animals into
notice. From 1669, when Malpighi described the anatomy and life-history
of the silkworm, a succession of what we now call biological types were
studied; among these were many invertebrates. Edmund King and John
Master contributed to Willis's treatise _De Anima Brutorum_ (1672)
the anatomies of the oyster, crayfish, and earthworm, all illustrated by
clear and useful plates. Heide (1683) wrote an account of the structure
of the edible mussel (Mytilus), in which mention is made of the ciliary
motion in the gill; Poupart (1706) and Méry (1710) wrote accounts of
the pond-mussel (Anodon). Swammerdam's elaborate studies of insects
and their transformations were followed up by a long succession of
memoirs by Frisch in Germany, Réaumur in France, and (shortly after
the close of the period now under discussion) De Geer in Sweden. The
extraordinary diligence and power of Swammerdam and Réaumur give a
very prominent place in the biology of the seventeenth and eighteenth
centuries to the structure and life-histories of insects. The great
generalisations of comparative anatomy do not belong to this period;
nevertheless, sagacious and luminous remarks are not wanting.
Adaptations of Plants and Animals: Natural Theology.
Natural adaptations and some of the problems which they suggest were
much studied during this period. Bock and Cesalpini had discussed still
earlier the mechanisms of climbing plants, aquatic plants, and plants
which throw their seeds to a distance. Swammerdam figured, not for the
first time, the sporangia and spores of a fern; Hooke the peristome of
a moss. The early volumes of the Académie des Sciences contain many
studies of natural contrivances. Perrault described the retractile claw
of the lion, the pointed papillæ on its tongue, the ruminant stomach
and the spiral valve of a shark's intestine. He improved upon Hooke's
account of the structure of a feather, and his magnified figures of
a bit of an ordinary quill and of a bit of an ostrich-plume might be
inserted into any modern treatise on animal structure.[7] Poupart
followed the later stages of the development of a feather. Méry gave a
minute yet animated description of the wood-pecker's tongue, explaining
how it is rendered effective for the picking up of insects, how it
is protruded and retracted, how it is stowed away when not in use.
Tournefort figured the oblique fibres of a leguminous pod, which he
called _muscles_, and showed how they twist the valves and squeeze
out the seeds.
Natural theology was much in the thoughts of the naturalists who
studied and wrote between 1660 and 1740. Ray discoursed upon the
_Wisdom of God as manifested in the Creation_. Swammerdam regularly
closed the divisions of his _Biblia Naturæ_ with expressions of
pious admiration. A long list of books expressly devoted to the same
theme might be given.[8] One weakness of the natural theologians was
their habit of looking upon the universe as existing for the convenience
of man. Still more fatal was the partiality with which they stated the
facts. While they dwell upon the adaptations which secure the welfare
of particular animals or plants, they are silent about the sufferings
caused by natural processes.
Spontaneous Generation.
During many ages every naturalist thought that he had ample proof of
the generation without parents of animals and plants. He knew that live
worms appear in tightly-closed flasks of vinegar; that grubs may be
found feeding in the cores of apples which show no external marks of
injury; and that weeds spring up in gardens where nothing of the sort
had been seen before. Certain kinds of animals and plants are peculiar
to particular countries; what more likely than that they should be the
offspring of the soil? Fables and impostures supported what all took
to be facts of observation. The great name of Aristotle was used to
confirm the belief that insects were bred from putrefaction; eels and
the fishes called Aphyæ from the mud of rivers. A belief in a process
of transmutation was often combined with a theory of spontaneous
generation. Francis Bacon not only held that insects were born of
putrefying matter, but that oak boughs stuck in the earth produced
vines.
Towards the end of the seventeenth century it occurred to one inquiring
mind that a particular case of spontaneous generation, which had
been accepted by everybody without hesitation, was capable of a less
mysterious explanation. Francesco Redi (1626-1698), physician to the
Duke of Tuscany, published in 1668 an account of his experiments on
the generation of blow-flies. He found that the flesh of the same
animal might yield more than one kind of fly, while the same fly might
be hatched from different kinds of flesh. He saw the flies laying
their eggs in flesh, and dissected eggs out of their ovaries. When he
kept off the flies by gauze the flesh produced no maggots, but eggs
were laid on the gauze. Redi concluded that flies are generated from
eggs laid by the females. He also studied insect-galls, and the worms
which feed on growing seeds. Like earlier observers, he was baffled by
finding live grubs in galls or nuts which were apparently intact, and
by the parasitic worms which are now and then found in the brain-case
and other closed cavities of quadrupeds. Such instances led him to
jump at the supposition of a "vivifying principle," which generated
living things of itself—a supposition contrary to the truer doctrine
which he taught elsewhere. Vallisnieri was able to explain how the egg
is introduced into the rose-gall, which a little later shows no mark
of injury; while Malpighi examined the young nut and found both hole
and egg. How parasitic worms reach the brain-case of the sheep could
be explained only in a later age. Meanwhile Swammerdam, Leeuwenhoek,
Réaumur, and many other special students confirmed and extended Redi's
experiments on the blow-fly; and every fresh instance of normal
generation in a minute organism did something to weaken the belief in
spontaneous generation.
Late in the eighteenth century that belief revived in a form less
easy of refutation. Leeuwenhoek had discovered that organic matter
putrefying in water often yielded abundance of microscopic organisms of
the most diverse kinds, many of which could resist drying in air and
resume their activity when moistened again. Buffon, ever ready with a
speculative explanation, maintained that such minute organisms were
spontaneously generated, and that they were capable of coalescing into
bodies of larger size and more complex structure. Needham supported
Buffon's theories by experiments. Taking infusions of meat, corking
them, and sealing them with mastic, he subjected them to a heat which
he thought intense enough to destroy life; after an interval the
microscope revealed an abundance of living things which he affirmed to
have been generated from dead matter. Spallanzani repeated Needham's
experiments with stricter precautions, sealed his flasks by fusing
their necks in a flame, and then immersing them in boiling water until
they were heated throughout. The infusions in such flasks remained
limpid; no scum formed on the surface; no bad smell was given off
when they were opened; and no signs of life could be detected by the
microscope. To meet the objection that the vegetative force of the
infusions had been destroyed by long heating he simply allowed air
to enter, when the micro-organisms quickly reappeared. Spallanzani's
methods, though far better than any which had been employed before, are
not quite unimpeachable, and could not be relied upon in an atmosphere
rich in germs; but they sufficed to create a strong presumption that
life is set up in infusions by germs introduced with the air.
This was by no means the end of the controversy, which broke out again
and again until it was laid to rest, whether finally or otherwise it
would be unwise to predict, by the experiments of Pasteur.
The Natural History of John Ray.
The sixteenth, seventeenth, and eighteenth centuries each possessed at
least one naturalist of wide learning and untiring diligence, who made
it his care to collect information concerning all branches of natural
history, to improve system, and to train new workers. Gesner, Ray, and
Linnæus occupied in succession this honourable position.
Ray was originally a fellow of Trinity College, Cambridge, who had
risen into notice by proficiency in academical studies. He then became
inspired by the hope of enlarging the knowledge of plants and animals,
and of producing what we should now call a descriptive fauna and flora
of Great Britain. His plan contemplated close personal observation,
travels at home and abroad, and the co-operation of pupils and friends.
[Illustration: JOHN RAY.
From an old engraving of the portrait by Faithorn.]
His chief assistant was Francis Willughby, a young man of wealth and
good family; while Martin Lister, a Cambridge fellow, who had already
laboured at natural history with good effect, undertook an independent
share in the work. Ray wisely began with what lay close at hand, and
published a catalogue of the plants growing around Cambridge. This was
not a mere list of species, but a note-book charged with the results
of much observation and reading. Journeys in quest of fresh material
were begun. Then Ray's well-laid scheme was disconcerted by calamities
which would have overwhelmed a less resolute man. He was driven from
Cambridge by the Act of Uniformity, and forced to serve for years as
a tutor in private families. When this servitude came to an end his
only livelihood was a small pension, bequeathed to him by Willughby,
on which he lived in rustic solitude. Willughby was cut off at the
age of thirty-six, having accumulated much information but completed
nothing. Lister became a fashionable physician, to whom natural history
was little more than an elegant diversion. The whole burden of the
enterprise fell upon Ray, who manfully bore it to the end. He completed
his own share of the work, prepared for the press the imperfect
manuscripts of Willughby, and before he died was able to fulfil the
pledge which he had given forty years before in the prosperity of early
manhood. It is needless to say that the natural history of Britain,
executed in great part by a poor and isolated student, fell far short
of what Ray might at one time have reasonably expected to accomplish.
Ray, like other early naturalists, saw that a methodical catalogue
of species, arranged on some principle which could be accepted in
all times and in all countries, was indispensable to the progress of
natural history, and such a catalogue formed an essential part of his
plan. Perhaps he was a little deficient in that discernment of hidden
affinities which has been the gift of great systematisers, but his
industry, learning, and candour accomplished much. Quadrupeds, birds,
reptiles, fishes, insects, and plants of every sort were reviewed by
him. British species naturally received special attention, but Ray
did not fail to make himself acquainted with the natural productions
of foreign countries, partly by his own travels, and partly by
comparing the descriptions of explorers. He seized every opportunity
of investigating the anatomy and physiology of remarkable animals and
plants, and attended to the practical uses of natural history. British
naturalists owed to him the first serviceable manuals for use in the
field.
Ray was the first botanist who formally divided flowering plants into
Monocotyledons and Dicotyledons. It was only natural that he should now
and then have misplaced plants whose general appearance is deceptive
(lily of the valley, Paris, Ruscus, etc.). He was perhaps the first
to frame a definition of a species; but here his success, as might be
expected, was not great. A species was with him a particular sort of
plant or animal which exactly reproduces its peculiarities generation
after generation. Any plant, for example, which comes up true from
seed, would according to Ray constitute a species. By this definition
many races of plants which are known to have been produced in nurseries
would rank as true species.
The Scale of Nature.
No one can closely examine a large number of plants and animals
without perceiving real or imaginary gradations among them. The
gradation, _shrews_, _monkeys_, _apes_, _man_,
is not very far from a real genealogical succession, confirmed by
structural and historical proofs. The gradation, _fish_,
_whale_, _sheep_, on the other hand, though it seemed equally
plausible to early speculators, is not confirmed by structure and
history. In the age of Aristotle and for long afterwards the ostrich was
believed to be a connecting link between birds and mammals, because it
possessed, in addition to obvious bird-like features, a superficial
resemblance to a camel (long neck, speed in running, desert haunts, and
a rather imaginary resemblance in the toes). Sedentary, branching
zoophytes were quoted as intermediate between animals and plants; corals
and barnacles as intermediate between animals or plants and stones.
Aristotle was convinced of the continuity of nature; his scale of being
extended from inanimate objects to man, and indicated, as he thought, the
effort of nature to attain perfection. Malpighi traced analogies between
plants and animals, identifying the seed and egg, as many had done before
him, assuming that viviparous as well as oviparous animals proceed from
eggs, and comparing the growth of metals and crystals with the growth
of trees and fungi. Leibnitz believed that a chain of creatures, rising
by insensible steps from the lowest to the highest, was a philosophical
necessity. Buffon accepted the same conclusion, and affirmed that every
possible link in the chain actually exists. Pope reasoned in verse
about a "vast chain of being," which reaches from God to man, and from
man to nothing. The eighteenth century was filled with the sound.
Bonnet in 1745 traced the scale of nature in fuller detail than had
been attempted before. He made Hydra a link between plants and
animals, the snails and slugs a link between mollusca and serpents,
flying fishes a link between ordinary fishes and land vertebrates, the
ostrich, bat, and flying fox links between birds and mammals. Man,
endowed with reason, occupies the highest rank; then we descend to the
half-reasoning elephant, to birds, fishes, and insects (supposed to be
guided only by instinct), and so to the shell-fish, which shade through
the zoophytes into plants. The plants again descend into figured stones
(fossils) and crystals. Then come the metals and demi-metals, which are
specialised forms of the elemental earth. Water, air, and fire, with
perhaps the æther of Leibnitz, are placed at the bottom of the scale.
In Bonnet's hands the scale of nature became an absurdity, by being
traced so far and in so much detail. It was not long before a reaction
set in. The great German naturalist, Pallas, in his _Elenchus
Zoophytorum_ (1766) showed that no linear scale can represent the
mutual relations of organised beings; the branching tree, he said,
is the appropriate metaphor. Cuvier taught that the animal kingdom
consists of four great divisions which are not derived one from
another, and his authority overpowered that of Lamarck, who still
maintained that all animals form a single graduated scale. A complete
reversal of opinion ensued, so complete that at length the theologians,
who had once seen in the scale of nature a proof of the wisdom of
Providence, were found fighting with all their might against the
insensible gradations which, according to Darwin's _Origin of Species_,
must have formerly connected what are now perfectly distinct forms of
life.
The eighteenth-century supporters of continuity in nature were not
merely wrong in picturing the organised world as a simple chain or
scale. They were also wrong in assuming that all the links or steps
still exist. We can now see that vast numbers are irrecoverably gone.
It is a safe prophecy that the filiation of species will never be
grasped by the intelligence of man except in outline, and even an
outline which shall truly express the genetic relations of many chief
types is unattainable at present.
The Sexes of Flowering Plants.