Chapter II: Part 2

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.

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.

As soon as men began to raise plants in gardens, or even earlier, they 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.

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]

Montesquieu (_Esprit des Lois_, 1748) was the first to exhibit on an impressive scale the power of the historical method. Natural development, determined by unalterable conditions, was with him the key to the right understanding of the past. It is well known that here and there a great thinker had before Montesquieu framed something like the same conception. The Politics of Aristotle[18] and Vico's study of the historical evolution of the Roman law (1725) are memorable anticipations. By 1748, the date of the _Esprit des Lois_, or 1749, the date of Buffon's first volumes, which come next before us, Newton's _Principia_ had made students of physics and astronomy practically familiar with the notion of universal causation.

Buffon's place in the history of science is that of one who accomplished great things in spite of weaknesses peculiarly alien to the scientific spirit. It was mainly he who, by strenuous exertions and largely at his own cost, transformed the gardens from which the king's physicians used to procure their drugs into what we now know as the Jardin des Plantes. By the untiring labours of fifty years he produced a Natural History in thirty-six volumes crowded with plates. Having won for himself a place side by side with Montesquieu and Gibbon, he employed it to direct attention to the larger questions of biology and geology. He was a pronounced freethinker, who promulgated bold views with a dexterity which saved him from condemnation by the theological tribunals. When his opinions were declared to be contrary to the teaching of the Church, he printed a conciliatory explanation, but never cancelled the passages objected to, which continued to appear in a succession of editions. His deficiencies, we must admit, were serious. He was a poor observer (partly because of short sight), and had no memory for small details. His enemies were able to taunt him with absurd mistakes, such as that cows shed their horns. He alienated the two foremost naturalists of the eighteenth century, Linnæus and Réaumur, by ignorant and scornful criticisms. His strong propensity to speculation, insufficiently checked by care to verify, might have brought him under the sarcastic remark of Fontenelle, that ignorance is less apparent when it fails to explain _what is_, than when it undertakes to explain _what is not_.

Buffon's fame is not seriously impaired by the fact that his great work is no longer read except by those who study the course of scientific thought. Few productions of the human intellect retain their value after a hundred years, and scientific treatises become obsolete sooner than others. It is consoling to recollect that, if their energy is quickly dissipated, it is at least converted into light.

In a history of biology Buffon is naturally a more important figure than Montesquieu. Buffon had imbibed evolutionary views from the _Protogæa_ of Leibnitz, which in turn made use of certain hypotheses of Descartes.[19] The _Histoire Naturelle_ inclines to some theory of evolution, especially in the later volumes. At first Buffon teaches that species are fixed and wholly independent of one another; some years later he is ready to believe that all quadrupeds may be derived from some forty original forms, while in a third and subsequent passage he puts the question whether all vertebrates may not have had a common ancestor. He does not shrink from saying that one general plan of structure pervades the whole animal kingdom—a belief that he could never have adequately supported by facts; Baer long afterwards (1828) searched in vain for evidence on this very point, while Darwin in 1859 admitted that his arguments and facts only proved common descent for each separate phylum of the animal kingdom;[20] he inferred _from analogy_ that probably all the organic beings which have ever lived on this earth have descended from some one primordial form.[21] Elsewhere Buffon makes bold to declare that Nature in her youthful vigour threw off a number of experimental forms of life, some of which were approved and adopted, while others were allowed to survive in order to give mankind a wider conception of her projects. There is generally some gleam of truth in Buffon's most fantastic speculations, but we often wish that he could have attended to the warning of Bossuet: "Le plus grand dérèglement de l'esprit est de croire les choses parce qu'on veut qu'elles soient."

Against all his shortcomings we must set the fact that Buffon strove to interpret the present by the past, the past by the present, geology by astronomy, geographical distribution by the physical history of the continents. One of his maxims expresses the fundamental thought of Lyell's _Principles of Geology_: "Pour juger de ce qui est arrivé, et même de ce qui arrivera, nous n'avons qu'à examiner ce qui arrive."

Hard-and-fast distinctions are the marks of imperfect theory. Early philosophers distinguished hot and cold, wet and dry, light and dark, male and female, as things different in kind. In later times organic and inorganic, animal and vegetable, the activities of matter and the activities of mind, have been sharply separated. But as knowledge increases these distinctions melt away; it is perceived that the extreme cases are either now connected by insensible gradations, or else spring historically from a common root. Hutton, Lyell, and their successors have made it clear that the history of the earth calls for no agents and no assumptions beyond those that are involved in changes now going on; the present is heir by unbroken descent to the past. Continuity has been established between all forms of energy. Even the chemical elements, once the emblems of independence, give indications that they too had a common origin. The nebular hypothesis, which has been steadily rendered more probable by the scientific discoveries of two centuries, traces all that can be perceived by the senses to a homogeneous vapour, and lays the burden of proof on those who believe that continuity has its limits. Every history, whether of planetary systems, or of the earth's crust, or of human civilisations, religions, and arts, is recognised as a continuous development with progressive differentiation.

Amateur Students of Living Animals.

A history of biology would be incomplete which took no notice of every-day observations of the commonest forms of life. Some of the best are due to the curiosity of men with whom natural history was no more than an occasional recreation. William Turner (a preacher, who became Dean of Wells), Charles Butler (a schoolmaster), Caius and Lister (physicians), Claude Perrault (a physician and architect), Méry and Poupart (surgeons), Frisch (a schoolmaster and philologue), Lyonet (an interpreter and confidential secretary), Roesel (a miniature painter), Henry Baker (a bookseller, who gained a competence by instructing deaf mutes), Leroy (ranger to the King of France), Stephen Hales, Gilbert White and William Kirby (country parsons), and William Spence (a drysalter) were all amateurs in natural history. To this list we might add Willughby, Ray, Leeuwenhoek, Réaumur, De Geer, Buffon, the Hubers, and George Montagu, who were either so fortunate in their worldly circumstances or so devoted to science as to make it their chief, or even their sole pursuit, though they did not look to it for bread. A large proportion of the naturalists whose names have been quoted occupied themselves with the habits and instincts of animals, and biology has been notably enriched by their observations. To Englishmen the most familiar name is that of Gilbert White, in whom were combined thirst for knowledge, exactness in description, and a feeling for the poetry of nature.

White used his influence to encourage what may be called _live natural history_, which, as he understood it, "abounds in anecdote[22] and circumstance." He bids his correspondents to "learn as much as possible the manners of animals; they are worth a ream of descriptions." His example has done more than his exhortations. He focusses a keen eye upon any new or little-known animal, such as the noctule, the harvest-mouse, or the mole-cricket; detects natural contrivances little, if at all, noticed before, such as the protective resemblance of the stone-curlew's young; dwells upon the practical applications of natural history, such as the action of earthworms in promoting the fertility of soils; and combines facts which a dull man would be careful to put into separate pigeon-holes, such as the different ways in which a squirrel, a field-mouse, and a nuthatch extract the kernels of hazel-nuts.

The many amateurs of the eighteenth century naturally demanded books written to suit them, and illustrated books with coloured plates, coming out in parts, found a ready sale. Some were devoted to insects, others to microscopic objects. In accordance with prevalent belief, the writers made a point of tracing the hand of Providence in the minutest organisms; many popular treatises were altogether devoted to natural theology. Some few of these natural history miscellanies contained original work, which has not yet lost its interest. The best is Roesel's _Insecten-belustigungen_ (four vols. 4to., 1746-61), memorable among other things for containing the original description of Amœba. For English readers Henry Baker wrote _The Microscope Made Easy_ (1743) and _Employment for the Microscope_ (1753).

Intelligence and Instinct in the Lower Animals.

The period with which we are now concerned (1741-1789) initiated the profitable discussion of the mental powers of animals. We are unable for lack of space to follow the investigation from period to period, and must condense into one short section whatever its history suggests.

In the year 1660 Aristotelians were still discoursing about the vegetative and sensitive souls which bridged the gulf between inanimate matter and the thinking man. Descartes had tried to prove that the bodies of men and animals are machines actuated by springs like watches. Man, however, according to Descartes, possesses a soul wholly different in its properties from his body, and apparently incapable of being acted upon by it. Man only can think; animals are capable only of physical sensations, and have no consciousness. Into speculations like these we shall not venture, being- content, like Locke, "to sit down in quiet ignorance of those things which upon examination are proved to be beyond the reach of our capacities." We shall merely note here and there facts ascertained by observation or experiment, and plain inferences drawn from such facts.

Swammerdam and Réaumur, besides many naturalists of less eminence, recorded a host of observations on the activities of insects. They contributed little to the discussion except new facts, for habit led them to ascribe without reflection every contrivance to the hand of Providence or else to Nature. Some of their facts, however, made a deep impression, none more than the exact agreement of the cells of the honeycomb with the form which calculation showed to be most advantageous.[23] The coincidence has lost some of its interest since the discovery that the theoretically best form of cell is hardly ever realised.[24] Réaumur,[25] in describing the process by which a certain leaf-eating caterpillar makes a case for itself out of the epidermis of an elm-leaf, showed that the caterpillar is not devoid of that kind of intelligence which adapts measures to circumstances. He cut off the margin where the upper epidermis of the leaf passes into the lower one, a margin which the insect had intended to convert into one side of its case; the caterpillar sewed up the gap. He cut off a projection which was meant to form part of the triangular end of the case; the caterpillar altered its plan, and made that the head-end which was originally intended to lodge the tail. This observation anticipates a better-known example taken from the economy of the hive-bee by Pierre Huber, which is mentioned below.

Buffon[26] heard with impatience all expressions of admiration for the works of insects. His poor eyesight and his repugnance to minutiæ disinclined him to pay much attention to creatures so small, and he had set himself up as the rival of Réaumur in physics and natural history. To pour contempt upon insects gratified both feelings at once. Bees, he said, show no intelligence at all; their actions are purely automatic, and their much-vaunted architecture is merely the result of working in a crowd. The cells of the honeycomb are hexagonal, not by reason of forethought or contrivance, but because of mutual pressure; soaked peas in a confined space form hexagonal surfaces wherever they touch.

The elder Huber seems to have denied to bees every trace of intelligence, but his son Pierre found it hard to go so far.[27] He remarked that the storage-cells of a honeycomb are not always exactly alike; they may be lengthened, cut down, or curved, when requisite. Cells which had been rudely trimmed with a knife were repaired with such dexterity and concert as to suggest that even the hive-bee has "le droit de penser." Bees would under compulsion build upwards or sideways, instead of downwards, as they like to do. Finding that they sought to extend their combs in the direction of the nearest support, he covered the support with a sheet of glass, on which they could get no footing. They swerved at once from the straight line, and prolonged their comb towards the nearest _uncovered_ surface, though this obliged them to distort their cells. He was driven to the conclusion that bees possess "a little dose of judgment or reason." In our own time, when all conscious adaptation of means to ends is believed to be worthy of the name of _reason_, it requires no great courage to ask why we deny such an attribute to all the lower animals.

In spite of examples like this, the favourite expression "blind instinct" helped to strengthen the conviction that the mental processes of animals are unsearchable. It is impossible to deny that the epithet _blind_ is appropriate in many cases. A bird will sit an addled egg all summer, or vainly but repeatedly attempt to make its tunnel in the insufficient breadth of a mud wall (Geositta). Of course such instances do not show that _all_ the acts of the lower animals are devoid of intelligence.