must have remarked that plants produce seeds, and that seeds develop
into new plants. The Greeks (Empedocles, Aristotle, Theophrastus)
recognised that the seed of the plant answers to the egg of the animal,
which is substantially though not literally true. None of the three
understood that a process of fertilisation always, or almost always,
precedes the production of seed. Had the date-palm, whose sexes
are separate, and which has been artificially fertilised from time
immemorial, been capable of cultivation in Greece, Aristotle would
not have said that plants have no sexes, and do not require to be
fertilised. His pupil, Theophrastus, knew only by hearsay of the male
and female date-palms, and affirmed that both bear fruit. Pliny, three
hundred years later, called pollen the fertilising substance, and gave
it as the opinion of the most competent observers that all plants are
of two sexes. The revivers of botany paid no attention to pollen or the
function of the flower; it is more surprising that in the following
century Malpighi, who had diligently studied the development of the
plant-embryo, should give so superficial an account of the stamen and
its pollen. About the same time Grew and Millington expressed their
conviction that "the attire" (anthers) "doth serve as the male, for
the generation of the seed."[9] A few years later Ray[10] speaks of
the masculine or prolific seed contained in the stamens. In 1691-4
Camerarius, professor at Tübingen, brought forward clear experimental
proof that female flowers, furnished only with pistils, produce seeds
freely in the neighbourhood of the male or staminate flowers, but
fail to do so when isolated. He distinctly inferred that the anthers
are male organs and the pistil the female organ. The claim set up on
behalf of Linnæus that he demonstrated, or helped to demonstrate,
the sexes of flowering plants has little foundation in fact. To make
out such details of the process of fertilisation as the formation of
pollen-tubes, the penetration of the ovules and the fusion of nuclei
required the improved microscopes of the nineteenth century.
The almost universal presence both in plants and animals of a process
of fertilisation is a fact whose physiological meaning we but
imperfectly grasp. Modern research has shown that the pollen-tube is
exceptional and confined to the flowering plants; the motile filament
of cryptogams, analogous to the spermatozoon of animals, is no doubt
a relatively primitive structure, which gives one of the strongest
indications of the common origin of all forms of life.
[6] Ray came at last to believe that fossils were the remains of actual
organisms, but he was still much hampered by his theological views.
[7] The second of the two has actually been so treated, but without
mention of Perrault's name.
[8] See Krause's _Life of Erasmus Darwin_.
[9] Grew's _Anatomy of Plants_, 1682.
[10] _Wisdom of God_, 1691.
PERIOD III.
1741-1789
Characteristics of the Period.
The chief historical events are the decline of the French monarchy, the
French revolution, the rise of Prussia, the expansion of England, and
the American Declaration of Independence. In the history of thought we
remark the introduction of the historical or comparative method, which
seeks to co-ordinate facts and to trace events to their causes. Science
steadily grows in influence, and freethought wins many triumphs; this
is the age of Voltaire, Rousseau, and the Encyclopædists, of David
Hume, of the French economists and Adam Smith.
Systems of Flowering Plants: Linnæus and the Jussieus.
Linnæus is remembered as a man of great industry, enterprise, and
sagacity, who was inspired from boyhood by a passion for natural
history and spent a long life in advancing it. He was early recognised
as a leader in more than one branch of the study.
L'Obel, Morison, and Ray had laboured to found a natural system of
flowering plants, and it was they who laid the foundation upon which
all their successors have built. The work did not, however, go steadily
forward on the original plan. When Linnæus entered upon the scene the
prevalent systems were only moderately natural, and far from convenient
in practice. To place the undescribed species which poured in from
North America and other distant countries was a difficult task,
with which the universities and botanic gardens of Europe could but
imperfectly cope. Linnæus, who had the instincts of a man of business,
saw that botany was falling into confusion, and that the only remedy
was a quick and easy method, which could be mastered in a few days and
applied with certainty. No such method, he well knew, could take into
account all the intricate affinities of plants, but to devise a perfect
method required the labours of generations of botanists; meanwhile
a temporary expedient, full of faults it might be, would remove a
pressing evil. Flowering plants had been arranged by the divisions
of the ovary, or by the petals and sepals, with no very satisfactory
results; it occurred to Linnæus to try the number of the stamens
and styles. Any such method was bound to present many anomalies,
associating plants which are only distantly related, and separating
plants which are closely related; but some of the worst anomalies were
avoided and some well-established families (Crucifers, Composites,
Labiates) retained at the expense of symmetry. Not even the pressing
need of simple definitions, which was allowed to spoil so natural a
group as the Umbellifers,[11] could induce Linnæus to place Ranunculus
and Potentilla in the same class.
Linnæus gained currency for his system by connecting it with the newly
accepted doctrine of sexes in plants. That doctrine was not conceived
nor demonstrated by him (see p. 48), and it had, as we now see, no
further connection with classification by stamens and styles than that
it explained the almost universal occurrence of such parts in flowering
plants. But Linnæus had persuaded himself that he had done more to
establish the existence of sexes in plants than anybody else, and that
the physiological importance of stamens and styles was a proof of their
systematic value. Neither of these beliefs can stand inquiry, but both
were extremely influential on contemporary opinion. The so-called
Sexual System achieved an immense success everywhere but in France and
Germany. Botanists of small experience were now able to say whether
the plants which seemed to be new were really undescribed or not;
if undescribed, what was their appropriate place in the system. The
congestion of systematic botany was relieved.
The great naturalist appealed to posterity by publishing the sketch of
a natural system of flowering plants, which he accompanied by judicious
expositions of the philosophy of classification. He had the permanent
reform of systematic botany really at heart; he did not believe that
his own Sexual System could be final; and he was glad to help in
setting up a better one. To this end he united groups of genera into
families which he did not pretend to define, being often guided only
by an obscure sense of natural bonds of union. Bernard de Jussieu, one
of the most patient and observant of systematists, devoted his life to
the same task, and profited by the example of Linnæus. He published
nothing, but found expression for his views in the arrangement of a
botanic garden at Versailles. His ideas were afterwards developed by
his nephew, A. L. de Jussieu, in the _Genera Plantarum_ (1789).
Affinity became at length the avowed basis of every botanical system.
No convenience in practice, no agreement or difference in habit, was
knowingly permitted to override this mysterious property. What then is
affinity? What are natural groups of animals and plants, and how do
they arise? Until the year 1859 no one could tell. The terse maxims
of Linnæus helped to guide naturalists into the right road, but a
single fact shows how inadequate they were. Linnæus emphatically and
repeatedly declared his belief in the constancy of species. But if
species were really constant, affinity between species must have been
no more than a delusive metaphor; the resemblances between distinct
species could not, on that supposition, be the effect of inheritance.
Linnæus' imperfect appreciation of the fundamental difference between a
natural classification of living things and such classifications as man
makes for his own practical ends is further revealed by his admission
of a third kingdom of nature.[12] Not only animals and plants, but
rocks and minerals as well, had, he thought, their genera and species.
The genus and species thereby become mere logical terms, independent of
inheritance and of life itself.
Linnæus had a passionate love of order and clearness, enforced by
an inexhaustible power of work. Hence he was able to serve his own
generation with great effect, to methodise the labours of naturalists,
to devise useful expedients for lightening their toil (such as his
strict binomial nomenclature),[13] and to apply scientific knowledge
to the practical purposes of life. But the complexity of nature is not
to be suddenly and forcibly reduced to order, and much of Linnæus'
work had to be done over again in a different spirit. Cuvier furnishes
a somewhat parallel case. Cuvier too was an indomitable worker. His
power of organisation moved the wonder of Napoleon, and there has been
no greater master of clear thought and clear expression. But, like
Linnæus, Cuvier overlooked much that was already obscurely felt and
clumsily worded by brooding philosophers, germs of thought which were
destined to become all-powerful in the course of a generation or two.
It must not be supposed that the labours of Linnæus and Cuvier were
bestowed in vain. All that was really valuable in their writings has
been saved, and biology will never forget how much it owes to their
life-long exertions.
[Illustration: CARL VON LINNÉ (CAROLUS LINNÆUS).
From an engraving (1779) after the portrait by Roslin.]
Réaumur and the History of Insects.
Réaumur was born to wealth, and made timely use of his leisure to study
the sciences and win for himself a place among natural philosophers.
His inclinations directed him first towards mathematics, physics, and,
a little later, towards the practical arts. He took a leading part in a
magnificent description of French industries, which had been undertaken
by the Académie des Sciences. Not content with describing the processes
in use, he perpetually laboured to improve them. The manufacture of
steel, tin-plate, and porcelain, the hanging of carriages and the
fitting of axles, the improvement of the thermometer, glass hives,
and the hatching of fowls' eggs by artificial heat are among the many
objects to which his attention was directed. Natural History gradually
took a more and more prominent place in his studies, and a great
_History of Insects_ engaged the last years of his busy life.
Réaumur was neither an anatomist nor a systematist, at least he
gained no distinction in either of these branches of biology. No
biological laboratory had been dreamt of in his day; he lacked the
manipulative skill of Swammerdam or Lyonet; he was no draughtsman,
and had to engage artists to draw for him. One qualification of
the first importance, however, he possessed in a high degree, the
scientific mind. As he watched the acts of an insect, questions at once
sagacious and practical suggested themselves in abundance, and these
questions he set himself to answer in the best possible way—viz.,
by observation and experiment. In close attention to the activities
of living things his ingenuity and patience found a boundless sphere
of exercise. Moreover all that he had seen he could relate in a
simple but picturesque manner, using the language familiar to the
best French society in the generation next after Madame de Sévigné.
Diffuse but clear, amusing but never frivolous, he won and kept the
attention of a multitude of readers, the best of whom were incited
to adopt his methods or to pursue inquiries which he had indicated.
His greatest successes were won in observing and interpreting the
natural contrivances of insects, the means by which they get their food
and provide for their safety; their transformations, instincts, and
societies. Kirby and Spence, which is still one of the best popular
accounts of insects in English, is largely based upon Réaumur; so
are other well-known treatises, in which the debt is less frankly
acknowledged. Réaumur greatly enlarged the knowledge of all kinds of
insects except the beetles and Orthoptera, which he did not live to
describe, and to this day his _Histoire des Insectes_ is a work of
fundamental importance, with which every investigator of life-histories
is bound to make himself acquainted.
No abstract of Réaumur's _Histoire des Insectes_ is possible, but
we may at least give one example of his mode of treatment. Let us select
his account of the proboscis of a moth, the first full account that
was ever given. He tells us that all moths have not an effective
proboscis, though he does not explain how some of them can dispense
with what seems so necessary an organ; this omission has been made good
by later entomologists. The proboscis, he goes on, springs from the
head, just between the compound eyes. When at rest, it takes up very
little room, for it is spirally rolled, like a watch spring; in some
cases it makes as few as one and a half or two turns, in others as
many as eight or ten; the base is often concealed by a pair of hairy
palps, which serve as feelers. Careful study of a moth as she flits
from flower to flower shows that she alights on the plant, unrolls her
proboscis, passes it into the corolla, withdraws it, perhaps coils it
for an instant, and then plunges it again into the tube. When this
manœuvre has been repeated several times, the moth flies off to another
flower.
Some moths have a tape-like proboscis; in others it is cylindrical. It
can be made to protrude by gentle pressure on the head, or be unrolled
by a pin passed into the centre of the spire; it is composed of
innumerable joints, and tapers from the base to the tip. When forcibly
unrolled, it often splits lengthwise into halves. At the time of escape
from the chrysalis the halves are always free, and they require careful
adjustment in order that a continuous sucking-tube may be obtained.
A newly emerged moth may be seen to roll and unroll its proboscis
repeatedly, until at last the halves cohere in the proper position.
Sometimes they begin to dry before the operation is completed, the
half-tubes get curled, and then the unfortunate moth becomes incapable
of feeding at all. Each half is a demi-canal, whose meeting edges
interlock by minute hooks. The mechanism reminds Réaumur of that which
connects the barbs of a feather; in both cases the hooks can be
adjusted rapidly and completely by stroking from base to tip, and in
both a water-tight junction is obtained. Besides the central canal,
along which fluids are sucked up, there are lateral canals (tracheæ)
filled with air.
Réaumur was careful to correct his anatomical studies by close
observation of the live insect. He reared an angle-shades moth, which
he kept several days without food. When he saw it repeatedly extending
its proboscis, he put near it a piece of sugar. The moth at once
began to suck, and became so absorbed in satisfying its hunger that
it allowed Réaumur to carry it on a sheet of paper to a window and to
examine it closely with a lens. The proboscis was sometimes extended
for several minutes at a time, and then rolled up for an instant; its
tip was either employed in exploring the surface or closely applied to
the sugar. By means of the lens a slender column of liquid was seen to
pass along the central canal towards the head. Now and then, however, a
limpid fluid was seen to pass down the proboscis; this was the saliva
which was used to moisten the sugar, and then sucked up again.
The Budding-out of New Animals (Hydra): another Form of Propagation
without Mating (Aphids).
In the year 1744 a young Genevese, Abraham Trembley, tutor in the
family of Bentinck, who was then English resident at the Hague, rose
into sudden fame by a solid and well-timed contribution to natural
history. Trembley and his pupils used to fish for aquatic insects in
the ponds belonging to the residence, and in the summer of 1740 he
happened to collect some water-weeds, which he put into a glass vessel
and set in a window. When the floating objects had come to rest, a
small green stalk, barely visible to the naked eye, was found attached
to one of the plants. From one end of the stalk filaments or tentacles
were seen to project, and these moved slowly about. When the vessel was
shaken the stalk and tentacles contracted, but soon extended themselves
again. Was this object a plant or an animal? Its shape and colour were
those of a plant, and sensitive plants were known which drooped when
touched or shaken. Further observation showed that it could move from
place to place, which favoured the animal interpretation. Trembley
determined to cut the stalk in two; if the halves lived when separated
the fact would favour the plant-theory. The halves at first gave no
signs of life beyond occasional contraction and expansion, but after
eight days small prominences were seen on the cut end of the basal
half. Next day the prominences had lengthened; on the eleventh day they
seemed to be growing into tentacles. Before long eight fully formed
tentacles were visible, and Trembley had two complete specimens in
place of one; both were able to move about.
After four years of observation a handsome quarto volume was published,
which told the history of "The freshwater Polyp," a name suggested
by Réaumur; the Latin name of Hydra was given by Linnæus. Hydra
had been discovered and slightly described forty years before by
Leeuwenhoek, who had seen two young polyps branching from one parent
and spontaneously becoming free. Trembley made out all that a simple
lens, guided by a skilful hand and a keen eye, could discover. Thirteen
plates were admirably engraved by another amateur, Pierre Lyonet,
who was in all respects a fit companion for Trembley. It was proved
that Hydra preyed upon living animals, especially upon the Daphnia
or water-flea. When it was well nourished it branched spontaneously
again and again, forming a compound mass made up of scores or even
hundreds of polyps, all connected with a single base. The power of
locomotion and the power of devouring prey were held to settle the
animal nature of Hydra, a decision to which zoologists have ever since
adhered. Lyonet went on to try the effect of division upon some common
freshwater worms, and found that each part grew into a complete worm.
Artificial division is not indispensable; in the worm called Nais
division takes place spontaneously at certain seasons, one segment
dividing repeatedly, so as to form the segments of a complete new
individual. The process may be repeated until a chain of worms is
produced, which at length breaks up.[14]
A nail was thus driven in a sure place. The conception of an animal was
enlarged, for it was shown that an animal may branch and multiply in
a way hitherto supposed to be peculiar to plants. The old connecting
links between animals and plants (zoophytes, sponges, etc.) had never
been really investigated; no one knew what sort of organisms formed or
inhabited their plant-like skeletons. But Hydra, thanks to Trembley's
description, furnished a clear example of an animal which possessed
some of the attributes of a plant. Forms more ambiguous than Hydra,
such as Volvox and Euglæna, were ultimately to make the distinction
between animal and plant very uncertain and shadowy. It was Hydra that
gave the first clue to the structure of the zoophytes, and dispelled
the false notion that corals are plants, bearing flowers, fruits, and
seeds.
[11] By associating with them a number of alien genera.
[12] The third kingdom of nature was taken from the alchemists.
[13] The binomial nomenclature had been gradually coming in ever since
the time of the Bauhins.
[14] This discovery is usually attributed to Bonnet, but the testimony
of Réaumur (_Hist. des Insectes_, Vol. VI., p. lvi.) and of Trembley
(_Hist. des Polypes d'eau douce_, p. 323) is decisive in favour of
Lyonet.
Baer[15] has remarked that Trembley's discovery appreciably modified
the teaching of physiology by showing that an animal without head,
nerves, sense-organs, muscles, or blood may perceive, feed, grow, and
move about.
At the time when Trembley was demonstrating the asexual propagation of
Hydra, Bonnet (supra, p. 45) was demonstrating the asexual propagation
of aphids. Both naturalists were natives of Geneva, and both, as well
as their associate Lyonet, were in a sense pupils of Réaumur, who
not only set them an admirable example, but directed their attention
to promising researches and discussed with them the conclusions
which might be drawn. Réaumur's experience had seemed to confirm
Leeuwenhoek's statement (supra, p. 34) that aphids produce young
alive, even though no males are to be found among them; but unlucky
accidents defeated his intention to confirm it by experiment, and when
Bonnet asked him to suggest a piece of work Réaumur gave him the aphid
problem.[16]
Bonnet filled a flower-pot with moist earth, introduced a food-plant
together with a single new-born aphid, and covered all up with a
bell-jar. In twelve days the aphid produced its first young one; in
a month ninety-five had been born from the same unfertilised parent.
As many as five generations were obtained without the intervention of
a male, each successive parent having been isolated from the moment
of its birth. It was, however, discovered, apparently by Lyonet, that
though viviparous reproduction without males went on regularly so long
as food was plentiful, males appeared towards the end of summer, and
fertilised the eggs which were destined to outlast the winter.
The aphids added a new and peculiar example to the known cases of
asexual propagation (plants and Hydra). Much discussion followed, but
the physiology of that age (and the same is true of the physiology of
our own age) was unable to reveal the full significance of the observed
facts. Insects have since furnished many instances of unfertilised eggs
which yield offspring. One such instance was already recorded, though
neither Leeuwenhoek, Réaumur, nor Bonnet knew of it. In the year 1701
Albrecht of Hildesheim placed a pupa in a glass vessel and forgot it.
A moth hatched out and laid eggs, from which a number of caterpillars
issued.
Lyonet, whom we have more than once had occasion to mention, afterwards
became celebrated as the author of one of the most laborious and
beautiful of insect-monographs. The structure of the larva of the
goat-moth was depicted by him in eighteen quarto plates, crowded with
detail.
The Historical or Comparative Method: Montesquieu and Buffon.
About the middle of the eighteenth century we remark the introduction
of a new, or almost new, method of investigation, which was destined to
achieve great results. Hitherto many men had been sanguine enough to
believe that they could think out or decide by argument hard questions
respecting the origin of what they saw about them. It was easier, but
not really more promising, to resort to ancient books which contained
the speculations of past generations of thinkers. Now at last men
set themselves to study what is, and by the help of historical facts
to discover _how it came to be_. The new method was first applied
to the institutions of human society, but was in the end extended to the
earth, life on the earth, and a multitude of other important subjects.
Most writers call this method _historical_, because history is
the chief means by which it seeks to trace causes. Others call it
_genetic_, because it goes back, whenever it can, to origins. It
might also be called _comparative_, because it compares, not only
things which are widely separated in time, but also things which are
separated in space, things which differ in form or tendency because
they have a common origin, and things which differ in origin because
they have a common form or tendency. Whether the institutions, arts,
and usages of mankind, or the species of plants and animals, are in
question, the study of history, together with the comparative study of
what now exists, results in increased attention to development, and
this again brings to light the continuity of all natural agents and
processes—continuity in time and continuity among co-existences. Since
the new method has succeeded in tracing the causes of many phenomena
which once seemed to obey no law, it has done much to strengthen the
belief in universal causation.
Down to the middle of the eighteenth century the book of Genesis had
been almost unanimously accepted in Europe as the only source of
information concerning the origin of the world, of man, of languages,
of arts and sciences. The whole duration of the world was restricted
to so brief a space that slow development was impossible, and it was
assumed that early history of every kind must be miraculous.[17]