Part 7

In 1787 Sprengel had remarked that the bases of the petals of Geranium silvaticum are beset with long hairs. Persuaded that no natural structure can be devoid of meaning, Sprengel asked what purpose these hairs might serve. A honey-gland in their midst suggested that they might protect the honey by keeping off the rain, which easily enters this shallow flower. Other honey-secreting flowers were found to possess mechanisms adapted to the same end. His first question suggested a second: Why should flowers secrete honey?

Malpighi had described the honey-glands of crown-imperial (1672), and had seen that the honey must be secreted by the petals, and not deposited from the atmosphere, according to the notion then current. Kölreuter (1761) had showed that insects may effect the pollination of flowers. Linnæus (1762) had given the name of _nectary_ to the honey-gland. He thought that the honey served to moisten the ovary, though he knew of staminate flowers furnished with nectaries. He also threw out the alternative conjecture that the honey is food for insects, which disperse the pollen by their wings. Sprengel improved upon all his predecessors, and made it clear that transference of pollen is the main purpose of the honey in flowers. He was put on the right track by the study of a forget-me-not flower. Here he found the honey protected from rain by the narrowness of the corolla-tube, whose entrance was almost closed by internal protuberances. The protuberances were distinguished by their yellow colour from the sky-blue corolla, and this conspicuous colouration led Sprengel to infer that insects might be thereby induced to seek for the store of honey within. He tested his conjecture by examining other honey-bearing flowers, and soon collected many instances of spots, lines, folds, and ridges, which might not only make insects aware of hidden stores of honey, but guide them to the exact place. Contrivances of the most diverse kinds, but all tending to invite the visits of insects and utilise them for the benefit of the plant, rewarded Sprengel's continued inquiries. He found that night-flowering plants, which could derive no advantage from coloured patterns, often have large white corollas, easily discerned in a faint light, and that these flowers give out an odour attractive to nocturnal insects. He found that the pollen-masses of an orchis are actually removed by large insects, though here no honey could be detected in the flower. Sprengel's fertility in probable conjecture is shown by his explanation of this puzzling case; he suggested that the orchis is a sham honey-bearer (Scheinsaftblume), which attracts insects by assuming the conspicuous size and coloration found in most honied flowers. Darwin suspected, and Herman Müller proved, that though the spur of the orchis-flower is empty, it yields when pierced a fluid attractive to bees and other insects. Sprengel discovered too how insects get imprisoned in the corolla of an Aristolochia, whose reflexed hairs allow small flies to creep in, but effectually prevent their escape until they have fertilised the pistils, when the hairs relax. These are only specimens of a multitude of adaptations which fill the book.

Sprengel insists upon the study of flowers under natural conditions; he could never have made out by the examination of plucked flowers how Nigella is fertilised. Flies with attached pollen-masses, which he found in spiders' nests, gave him the hint as to the way in which the fertilisation of orchids is effected. Definite questions must be put if observation is to be profitable. What is the use of honey to the plant—of this coloured spot—of these hairs? He notes the peculiarities of wind-fertilised and insect-fertilised flowers, the relative abundance of the pollen, the form of the stigma, the presence or absence of honey, the size, colour, and scent of the corolla. Here is a pretty illustration from his pages. Pluck a branch of hazel, aspen, or alder, with unexpanded catkins, and also one from the male sallow; place them in water, and keep them in a sunny window until the anthers are ripe. A vigorous puff will then discharge a cloud of pollen from the _wind-fertilized_ catkins, but none from the _insect-fertilised_ catkin of the sallow. What Linnæus said about the flowers of trees appearing before the leaves, in order that the pollen may more easily reach the stigmas, holds good, Sprengel remarks, only of wind-fertilised trees. The lime, which is insect-fertilised, flowers in the height of summer, when all the branches are crowded with leaves.

Sprengel left it to later biologists to complete his discovery. "That wonderfully accurate observer, Sprengel," says Darwin,[32] "who first showed how important a part insects play in the fertilisation of flowers, called his book _The Secret of Nature Displayed_; yet he only occasionally saw that the object for which so many curious and beautiful adaptations have been acquired, was the cross-fertilisation of distinct plants; and he knew nothing of the benefits which the offspring thus receive in growth, vigour, and fertility." Not even

Cuvier and the Rise of Palæontology.

If this historical sketch had been prepared within a few years of the death of Cuvier, it would no doubt have held him up as the greatest of zoologists and comparative anatomists. Nor would it have been hard to find reasons for such a verdict. His _Règne Animal_ extended and corrected the zoological system of Linnæus; his comparative anatomy, and especially his comparative osteology, were far ampler and more exact than anything that had been attempted before. It would not have been forgotten, moreover, that he was the practical founder of the new science of palæontology.

At a later time, say in the sixties and seventies of the nineteenth century, when the Origin of Species controversy was in full blast, any estimate of Cuvier by an evolutionist would have been much less laudatory. Cuvier had actively opposed that form of evolution which had been brought forward in his day, and with such power as to close the discussion for a time. The assailants of the Origin of Species found his refutation of unity of type and progressive development adaptable to the new situation, and the reasoning which had pulverised Geoffrey St. Hilaire was brought out again in order to pulverise Darwin. Then the supporters of Darwin found it necessary to show that Cuvier was by no means infallible. This they were able to do without introducing matter foreign to the main question, for Cuvier's exposition of fixity of species, of the principles of classification and of the process of extinction, were entirely opposed to the beliefs not only of Darwin, but of Lyell and the whole school which stood out for historical continuity, treated history of every kind as a process of development, extended almost without limit the duration of life on the earth, and enforced the obvious but neglected truth that results of any magnitude whatever may proceed from small causes operating through a sufficient length of time.

Darwin's main contentions are now accepted by the scientific world, and Cuvier's hostility to particular forms of evolution has become a mere historical episode of no lasting importance. Angry disputes concerning the weight of his authority are at an end; he is not to be blamed because thirty years after his death he was set up as judge of a cause which he had not heard. We are now ready to make fair allowance for the time in which his lot was cast—an age when geology, embryology, palæontology, and distribution were mere infants, some of them hardly yet born. We can also admit without reserve the incompetence of certain of Cuvier's antagonists, and justify the severity with which he treated unity of type as stated and defended by Geoffroy St. Hilaire. Now that the dust of controversy has settled, we are chiefly concerned to inquire: What of all Cuvier's work has proved to be really permanent? His zoology and his comparative anatomy have had to be completely re-cast, partly because of the new light thrown on them by embryology and the doctrine of descent with modification. His studies of extinct vertebrates, however, called into existence a new science, the science of Palæontology,[33] and it is mainly this which gives him a lasting and honoured place in the history of biology.

At the end of the eighteenth century it had been rather grudgingly admitted that some few animals were actually extinct. Buffon was able to quote as indubitable examples the mammoth and the mastodon. Their occurrence in countries unknown to the ancients, such as Siberia and North America, disposed of the explanation long clung to by the learned—viz., that their bones were the remains of elephants which had been led about by the Roman armies, while their large size and the ease with which they can be recognised rendered it highly improbable that they still survived anywhere on the surface of the globe.

It was therefore natural that Cuvier's first study in palæontology should relate to extinct elephants. He compared and distinguished several species, showed that they were distinct from the existing Asiatic and African species, a fact which had escaped the notice of Pallas, and argued from the well-known case of a Siberian mammoth preserved in ice and frozen mud with hardly any decomposition that it must have been overwhelmed by a sudden "revolution of the earth." Whatever we may think of Cuvier's geology, his comparisons of all known elephants, recent and fossil, introduced a new standard of exactness into these inquiries. From this beginning he went on to study all the extinct vertebrates which he could discover in public or private collections. By 1821 he had published elaborate and well-illustrated descriptions of near a hundred extinct animals, an extraordinary output for one investigator.

The most remarkable of his palæontological discoveries were made at home, in the lower tertiary rocks which underlie the city of Paris. He proved that in the valley of the Seine a large population of animals, all now extinct, had formerly flourished. None of these discoveries impressed his contemporaries more than the celebrated case of the fossil opossum. The bones were imbedded in a slab of gypsum, and were at first imperfectly exposed. The lower jaw, however, exhibited a peculiarity of marsupial or pouched animals, for its angle had an inwardly projecting shelf, not found in other quadrupeds. The opossums, like all marsupial animals, bear on the front of the pelvis two long bones, which support the pouch. These were as yet concealed, and Cuvier delayed clearing them until he had summoned friends, some of whom may have been sceptical about the possibility of reasoning with certainty from anatomical data. Warning them what to expect, he removed with a sharp tool the film of stone, and revealed the long and slender marsupial bones.[34] The ancient existence of marsupials in France was then a striking and almost incredible fact; increase of knowledge has not lessened its interest, though it has abated some of the wonder.

The fossil ungulates (hoofed quadrupeds) of the Paris basin taxed Cuvier's patience and skill to the utmost. In the tiresome work of piecing together a multitude of imperfect skeletons he set an example to all future palæontologists. That he drew general conclusions which we are unable to accept, and failed to draw conclusions which seem obvious to us, will surprise nobody whose reading has taught him how unprepared were the biologists of that age to handle great questions concerning the origin and extinction of races. Cuvier recognised among the fossils of the Paris quarries the bones of two genera of ungulates very different from any of recent times. One resembled the rhinoceros, tapir, and horse in being odd-toed; this he called Palæotherium. Another had the hind-foot even-toed, like a ruminant, though the fore-foot, with which he was imperfectly acquainted, showed points of resemblance to the other group. How cautiously he did his work may be gathered from the fact that he spent fifteen years upon the collection of facts before he attempted to restore these extinct forms, though almost every bone in their bodies had during that time passed through his hands.

The great interest of these fossil ungulates to the modern biologist is that they are relatively primitive types of the order. Palæotherium is not far from the ideal common ancestor of the rhinoceros, tapir, and horse; Anoplotherium not altogether unlike the ideal common ancestor of the hippopotamus, the swine, and the ruminants. It has been suspected that Cuvier was less obstinately devoted to the tenet of fixity of species than he was willing to admit in public. Whatever his private leanings may have been, he stood out resolutely for cogent proofs of transmutation. When it was contended that the Palæothere might have been the remote ancestor of existing ungulates, he demanded that the intermediate links should be produced. His demand could not be met till many years later, though intermediate forms between the Palæothere and the horse have since been furnished in abundance. Reserve about far-reaching deductions was surely wise at a time when plausible speculation was rife, and we ought not to judge Cuvier severely for having aspired to a rigour unattainable in a natural science, and certainly not always observed by himself. He hoped to see biology become as exact as astronomy. The hope may have been chimerical, but emphasis on this side was not altogether out of place in the generation of Geoffroy St. Hilaire and Oken.

If the great master who laid the foundations of palæontology could revisit the scene of his former labours, he would find that many strange things had happened since the appearance of his _Ossemens Fossiles_. He would perhaps be stupefied at first to discover how little is now made of the Revolutions of the Earth, the proofs of which had seemed to him unimpeachable, while the conjectures about the development of new races, which in his own day had been almost negligible, have proved to be anticipatory of fundamental biological truths. The first shock over, one can imagine the zest with which he would strive to combine the familiar facts into a body of new doctrine. The ungulates, recent and fossil, would of course interest him particularly. He would recognise the gradations of structure which run through the whole order, branching and crossing in all directions; gradation in the number of the toes, in the rearing of the body more and more upon the toe-tips, in the progressive complication of the teeth. One chain of examples would lead from the shallow, tuberculate molar of the pig to the molar of the horse or ruminant, deep and massive, with crescentic enamel-folds; another would illustrate the gradual development of tusks from ordinary incisors or canines; a third series would show the steps by which the primitive ungulate dentition became reduced to the dentition of the elephant, with only a single pair of incisors, enlarged into tusks several feet long, with no canines but molars of great weight, complicated by extreme folding. It would surprise and delight him to compare the almost insensible steps by which his own Palæothere can be seen to pass into the modern horse. Then we can imagine how our regenerate Cuvier would draw nearer and nearer to the common ancestor of the whole group, a five-toed, plantigrade ungulate, with the full dentition of forty-four unspecialised teeth, and how readily he would admit that Phenacodus, both in its structure and its geological horizon, was just the common ancestor that theory required. The proofs of intermediate stages between ancient and modern ungulates which he had once called for in vain, he would now find ready to his hand. It might well seem that the history of the ungulates, with all its modern expansions, would suffice to occupy even his unparalleled energy. He would see with delight how the palæontology which he had been the first to treat as a science has enlarged the comparative anatomy of which also he was so great a master. He would cheerfully admit that both yield proofs of that doctrine of descent with modification which a hundred years ago seemed to him so questionable.

Chamisso on the Alternation of Generations in Salpa.

Trembley (see p. 57) had shown that Hydra, though an animal, multiplies by budding like a plant. He got indications, upon which he did not altogether rely, that it also propagated by eggs, and ten years later (1754) this supposition was confirmed by Roesel, who figured the egg, though he was unable to demonstrate that a young Hydra issues from it; subsequent inquiry has placed the fact beyond doubt. In 1819 Chamisso announced that Salpa, a well-known Tunicate which abounds at the surface of the sea, exhibits a regular alternation of the two modes of increase, the egg-producing form being succeeded by a budding form, the budding form by the egg-producing form, and so on indefinitely. Sars a few years later showed that the common jelly-fish Aurelia also propagates by eggs and buds alternately. Here the familiar swimming disks, which are of two sexes, produce eggs from which locomotive larva issue. The larva at length settles down and takes a Hydra-like form. It pushes upwards an ascending column, which divides transversely and forms a pile of slices, each destined to become a free, sexual Aurelia. The alternation of generations may be regarded as resulting from the introduction of budding into the early stage of a life-history which culminates in sexual reproduction, much as if a caterpillar were to divide repeatedly and form more caterpillars, each of which ultimately became a moth. The case which has been given as an illustration actually occurs in nature. A parasitic caterpillar, that of Encyrtus, divides while still an embryo, so that one egg produces several moths.[35] Many other cases of alternation have since been found among animals, and it seems to be the rule among plants.

Alternation of generations may be complicated by association with transformation, by the omission of stages usual in the class, and by budding-out from one part instead of from the whole body. In particular cases the complication becomes so great that biological language breaks down under it. Such terms as _generation_, _individual_, _organ_, _larva_, _adult_, cannot always be used consistently without either being strained or artificially limited.

Baer and the Development of Animals.

The curiosity of the ancient Greeks led them to look for the chick within the egg, and Aristotle mentions the beating of the heart as a thing which might be observed in a third-day embryo. After the revival of science Fabricius of Acquapendente figured the chief stages of development, from the first visible rudiments to the escape from the egg-shell. Harvey, the discoverer of the circulation, not only studied the developing chick, but took advantage of the rare opportunity of dissecting breeding does from the royal parks. His treatise on Generation is unfortunately impaired by Aristotelian philosophy, and some of the theories there set forth gave much trouble to Swammerdam. The oft-cited maxim "Omne vivum ex ovo" does not occur exactly in this form in Harvey's writings,[36] nor does it fairly state his own belief. Those who read his _De Generatione_ will see that his knowledge was insufficient to justify so wide a generalisation; on this head it is enough to mention that he was persuaded of the production of insects without parents from putrefying matter.[37]

Malpighi was the first to apply the microscope to the embryonic chick. His figures are surprisingly full of interesting detail, and so far in advance of their age that they long failed to produce their due effect. On one point Malpighi unconsciously led naturalists astray for a hundred years or more. On examining a fowl's egg which he supposed to be unincubated, he discovered within it an early embryo. From this he concluded that the embryo _pre-exists_ in the egg, like a plant-embryo in a seed. He mentions one circumstance which makes everything intelligible. The egg was examined in August, during a time of great heat, and the Italian summer no doubt started development, like the hot sand of Aden, in which Chinamen hatch their eggs. Swammerdam too enforced the same belief in pre-existing germs. From the fact that the butterfly can be revealed by opening the skin of a full-fed caterpillar he inferred (quite contrary to the opinion which he expresses elsewhere) that one animal had formed inside another. This led him to say that there is no such thing as generation in nature, but merely the expansion of germs which lie enclosed one within another. By his theory he explained how Levi could pay tribute to Melchizedek before he was born, and how the sin of Adam can be laid to the charge of all his posterity. The belief in the pre-existence of germs was first shaken by Caspar Wolff (see p. 81), who examined unincubated eggs but found no germ which could be detected by the histological methods then employed.

Swammerdam's _Biblia Naturæ_ contains useful figures of early and late tadpoles; in particular, he describes a stage in which the body is entirely composed of rounded "lumps" or "granules," the _cells_ of modern biology.

Early in the nineteenth century Pander and Baer, both of whom were pupils of Döllinger, a teacher of extraordinary influence, gave a new impetus to the study of development. Pander (1817-8) published an account of the early stages of the chick, illustrated by beautiful plates by D'Alton. Baer (1828-37) carried the work much further, not only greatly extending the knowledge of the developing chick, but discovering the mammalian ovum (1827), and announcing generalisations which down to 1859 were the most luminous that embryology had ever furnished; we may call him the founder of comparative embryology. He shows that development may supply decisive indications of the zoological position of animals; it teaches, for instance, that insects are of higher grade than arachnids or crustaceans, and that amphibians ought not to be united with reptiles. He describes the development of an animal as a process of differentiation, the general becoming special, and the homogeneous heterogeneous; differentiation is, he remarks, the law under which not only animals but solar systems develop. He maintains that the embryo, though gradually attaining complexity, makes no transition to a different type—_e.g._, the vertebrate is never in any stage anything but a vertebrate. All animals, he believes, are probably at first similar, and take the form of a hollow sphere (the _gastræa_ of modern embryology). There are, he says, no new formations in nature; all is conversion. When he comes to speak of the pharyngeal clefts of mammals and birds, recently discovered by Rathke, he remarks that their correspondence with the gill-clefts of fishes is obvious. We wonder what is coming next, but our curiosity is not gratified by any memorable deduction. Neither here nor in his miscellanies (_Reden_), published nearly fifty years later, does he admit that mammals and birds can have descended from gill-breathing vertebrates. If we are inclined to hint that Baer, having gone so far, might well have gone a little farther, it is only fair to recollect that every leader in science is more or less open to the same reproach.

The Cell Theory.