Leeuwenhoek worked with simple lenses, ground and mounted by his own hands. It was easy to make lenses of high magnifying power, but hard to correct their optical defects, to bring a sufficiently strong light to bear upon the object, and to focus the lens. When he wished to send out his preparations for examination by others, he found it best to fix the objects in the focus, and to provide each with a separate lens. With such microscopes he managed to study and figure very minute objects, such as blood-corpuscles, spermatozoa, and bacteria. The spermatozoa were brought under his notice by a young Dutch physician named Hamm; but it was Leeuwenhoek's account of them, and his daring theory of their physiological rÔle, which gave them such celebrity. To Leeuwenhoek we owe the first discovery of the rotifers, the infusoria, Hydra, the yeast-cell, the bacteria, and the generation of aphids without male parents.

The tradition of the minute anatomists has never been lost, though we shall be unable to pursue it in these pages. Lyonet (see p. 61) even surpassed Swammerdam in the elaborate finish of some of his insect-dissections.

Early Notions about the Nature of Fossils.

Throughout the sixteenth century naturalists held animated debates about the shells which are found far from the sea, and even on the top of high hills. Had they ever formed part of living animals or not? Such a question could hardly have been seriously discussed among simple-minded people; but the learned men of the sixteenth century were rarely simple-minded. They had been trained to argue, and argument could make it plausible that such shapes as these were generated by fermentation or by the influence of the stars. So prevalent were these doctrines that it entitles any early philosopher to the respect of later generations that he should have taken shells, bones, and teeth to be evidences of animal life. In this singular roll of honour we find the names of Cesalpini, Palissy, Scilla, Stenson, Hooke, and Woodward.

In England the struggle between philosophy and common-sense was long kept up. Dr. Ralph Cudworth of Cambridge taught that there is in nature a subordinate creative force of limited power and wisdom, to whose imperfections may be attributed the "errors and bungles" which now and then mar the work. To this subordinate creative force he gave the name of "vegetative soul," or "plastic nature." None but Cambridge men, it would appear, felt the weight of Cudworth's reasoning; but several of these, and especially John Ray[6] and Martin Lister, defended his conclusions in published treatises. Lister, in a chapter devoted to "cochlites," or shell-shaped stones, pointed out that they differ from true shells in being of larger size, in occurring far from the sea, in being formed of mere stony substance, and in being often imperfect. Some naturalists had conjectured that the living animals of the cochlites still exist at great depths in the sea, but Lister evidently thought otherwise.

In the eighteenth century the belief that fossils are the remains of actual animals and plants more and more prevailed, the death and sealing up of the organisms being generally attributed to Noah's flood. The occurrence of fossils on high mountains seemed so strong a confirmation of the Biblical narrative that Voltaire was driven to invent puerile explanations in order to dispel an inference so unwelcome to him. By the end of the century most naturalists accepted the doctrine that the great majority of fossils are the remains of organisms now extinct—a doctrine which was enforced by the remarkable discoveries of Cuvier (see p. 93). Nearly at the same time William Smith established the important truth that almost every fossil marks with considerable precision a particular stage in the earth's history.

Comparative Anatomy: the Study of Biological Types.

Between 1660 and 1740 the scope of natural history became sensibly enlarged. System had been hitherto predominant, but the systems had been partial, treating the vertebrate animals and the flowering plants with as much detail as the state of knowledge allowed, but almost ignoring the invertebrates and the cryptogams. System was now studied more eagerly than ever by such naturalists as Ray and LinnÆus, but new aspects of natural history were considered, new methods practised, new groups of organisms included. Many remarkable vertebrates were anatomically examined for the first time. Claude Perrault and his colleagues of the AcadÉmie des Sciences dissected animals which had died in the royal menagerie, and compared the parts and organs of one animal with those of another; Duverney compared the paw of the lion with the human hand; in England Tyson studied the anatomy of the chimpanzee, porpoise, opossum, and rattlesnake, searching everywhere for the transitions which he believed to connect all organisms, and to form "Nature's Clew in this wonderful labyrinth of the Creation." The new microscopes helped to bring the lower and smaller animals into notice. From 1669, when Malpighi described the anatomy and life-history of the silkworm, a succession of what we now call biological types were studied; among these were many invertebrates. Edmund King and John Master contributed to Willis's treatise De Anima Brutorum (1672) the anatomies of the oyster, crayfish, and earthworm, all illustrated by clear and useful plates. Heide (1683) wrote an account of the structure of the edible mussel (Mytilus), in which mention is made of the ciliary motion in the gill; Poupart (1706) and MÉry (1710) wrote accounts of the pond-mussel (Anodon). Swammerdam's elaborate studies of insects and their transformations were followed up by a long succession of memoirs by Frisch in Germany, RÉaumur in France, and (shortly after the close of the period now under discussion) De Geer in Sweden. The extraordinary diligence and power of Swammerdam and RÉaumur give a very prominent place in the biology of the seventeenth and eighteenth centuries to the structure and life-histories of insects. The great generalisations of comparative anatomy do not belong to this period; nevertheless, sagacious and luminous remarks are not wanting.

Adaptations of Plants and Animals: Natural Theology.

Natural adaptations and some of the problems which they suggest were much studied during this period. Bock and Cesalpini had discussed still earlier the mechanisms of climbing plants, aquatic plants, and plants which throw their seeds to a distance. Swammerdam figured, not for the first time, the sporangia and spores of a fern; Hooke the peristome of a moss. The early volumes of the AcadÉmie des Sciences contain many studies of natural contrivances. Perrault described the retractile claw of the lion, the pointed papillÆ on its tongue, the ruminant stomach and the spiral valve of a shark's intestine. He improved upon Hooke's account of the structure of a feather, and his magnified figures of a bit of an ordinary quill and of a bit of an ostrich-plume might be inserted into any modern treatise on animal structure.[7] Poupart followed the later stages of the development of a feather. MÉry gave a minute yet animated description of the wood-pecker's tongue, explaining how it is rendered effective for the picking up of insects, how it is protruded and retracted, how it is stowed away when not in use. Tournefort figured the oblique fibres of a leguminous pod, which he called muscles, and showed how they twist the valves and squeeze out the seeds.

Natural theology was much in the thoughts of the naturalists who studied and wrote between 1660 and 1740. Ray discoursed upon the Wisdom of God as manifested in the Creation. Swammerdam regularly closed the divisions of his Biblia NaturÆ with expressions of pious admiration. A long list of books expressly devoted to the same theme might be given.[8] One weakness of the natural theologians was their habit of looking upon the universe as existing for the convenience of man. Still more fatal was the partiality with which they stated the facts. While they dwell upon the adaptations which secure the welfare of particular animals or plants, they are silent about the sufferings caused by natural processes.

Spontaneous Generation.

During many ages every naturalist thought that he had ample proof of the generation without parents of animals and plants. He knew that live worms appear in tightly-closed flasks of vinegar; that grubs may be found feeding in the cores of apples which show no external marks of injury; and that weeds spring up in gardens where nothing of the sort had been seen before. Certain kinds of animals and plants are peculiar to particular countries; what more likely than that they should be the offspring of the soil? Fables and impostures supported what all took to be facts of observation. The great name of Aristotle was used to confirm the belief that insects were bred from putrefaction; eels and the fishes called AphyÆ from the mud of rivers. A belief in a process of transmutation was often combined with a theory of spontaneous generation. Francis Bacon not only held that insects were born of putrefying matter, but that oak boughs stuck in the earth produced vines.

Towards the end of the seventeenth century it occurred to one inquiring mind that a particular case of spontaneous generation, which had been accepted by everybody without hesitation, was capable of a less mysterious explanation. Francesco Redi (1626-1698), physician to the Duke of Tuscany, published in 1668 an account of his experiments on the generation of blow-flies. He found that the flesh of the same animal might yield more than one kind of fly, while the same fly might be hatched from different kinds of flesh. He saw the flies laying their eggs in flesh, and dissected eggs out of their ovaries. When he kept off the flies by gauze the flesh produced no maggots, but eggs were laid on the gauze. Redi concluded that flies are generated from eggs laid by the females. He also studied insect-galls, and the worms which feed on growing seeds. Like earlier observers, he was baffled by finding live grubs in galls or nuts which were apparently intact, and by the parasitic worms which are now and then found in the brain-case and other closed cavities of quadrupeds. Such instances led him to jump at the supposition of a "vivifying principle," which generated living things of itself—a supposition contrary to the truer doctrine which he taught elsewhere. Vallisnieri was able to explain how the egg is introduced into the rose-gall, which a little later shows no mark of injury; while Malpighi examined the young nut and found both hole and egg. How parasitic worms reach the brain-case of the sheep could be explained only in a later age. Meanwhile Swammerdam, Leeuwenhoek, RÉaumur, and many other special students confirmed and extended Redi's experiments on the blow-fly; and every fresh instance of normal generation in a minute organism did something to weaken the belief in spontaneous generation.

Late in the eighteenth century that belief revived in a form less easy of refutation. Leeuwenhoek had discovered that organic matter putrefying in water often yielded abundance of microscopic organisms of the most diverse kinds, many of which could resist drying in air and resume their activity when moistened again. Buffon, ever ready with a speculative explanation, maintained that such minute organisms were spontaneously generated, and that they were capable of coalescing into bodies of larger size and more complex structure. Needham supported Buffon's theories by experiments. Taking infusions of meat, corking them, and sealing them with mastic, he subjected them to a heat which he thought intense enough to destroy life; after an interval the microscope revealed an abundance of living things which he affirmed to have been generated from dead matter. Spallanzani repeated Needham's experiments with stricter precautions, sealed his flasks by fusing their necks in a flame, and then immersing them in boiling water until they were heated throughout. The infusions in such flasks remained limpid; no scum formed on the surface; no bad smell was given off when they were opened; and no signs of life could be detected by the microscope. To meet the objection that the vegetative force of the infusions had been destroyed by long heating he simply allowed air to enter, when the micro-organisms quickly reappeared. Spallanzani's methods, though far better than any which had been employed before, are not quite unimpeachable, and could not be relied upon in an atmosphere rich in germs; but they sufficed to create a strong presumption that life is set up in infusions by germs introduced with the air.

This was by no means the end of the controversy, which broke out again and again until it was laid to rest, whether finally or otherwise it would be unwise to predict, by the experiments of Pasteur.

The Natural History of John Ray.

The sixteenth, seventeenth, and eighteenth centuries each possessed at least one naturalist of wide learning and untiring diligence, who made it his care to collect information concerning all branches of natural history, to improve system, and to train new workers. Gesner, Ray, and LinnÆus occupied in succession this honourable position.

Ray was originally a fellow of Trinity College, Cambridge, who had risen into notice by proficiency in academical studies. He then became inspired by the hope of enlarging the knowledge of plants and animals, and of producing what we should now call a descriptive fauna and flora of Great Britain. His plan contemplated close personal observation, travels at home and abroad, and the co-operation of pupils and friends.

John Ray.

From an old engraving of the portrait by Faithorn.