Part 4

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.