Chapter IV: Part 4

Any one of the higher animals or plants admits of analysis into _organs_, each adapted to one or more functions. Bichât (1801) showed that the body of one of the higher animals is not only a collection of organs, but also a collection of _tissues_, and the same is true of the higher plants. Analysis of the organism was carried a step further when in 1838-9 Schleiden and Schwann announced that all the higher animals and plants are made up of _cells_, which were at first supposed to consist in every case of a cell-wall, fluid contents, and a nucleus.[38] It was soon discovered that the cell-wall is as often absent as present, and that the cell-contents are not simply fluid; the nucleus is still believed to be universal. Schwann proved that nails, feathers, and tooth-enamel, though not obviously cellular, consist of nothing but cells, and it was afterwards shown that bone, cartilage, fatty tissue, and fibrous tissue arise by the activity of cells which disappear from view in the abundance of their formed products. The individual cells of a complex organism are usually themselves alive; sometimes, as in ciliated epithelium, they give indications of life longafter they have been separated from the body. The preponderating importance of the transparent jelly or protoplasm became clear when it was recognised that this alone is invariably present, and that this alone responds to stimuli. The nucleus is believed to be only a specialised part of the cell-protoplasm.

The cell-theory, like nearly every theory, was neither altogether new nor in its first form altogether complete. Before 1838 cell-division, as we should now call it, had been indistinctly seen to be the process by which the body of one of the higher animals is built up. Leeuwenhoek and Swammerdam had found a wholly cellular stage in frog-embryos (see p. 103), while Prévost and Dumas in 1824 had in effect discovered that the cells of which such embryos consist result from repeated division of an egg; Mohl in 1835 observed the actual division. Even Schwann, however, was not acquainted with the important fact that every cell arises by the division of a pre-existing cell.

Swarm-spores of algæ showed that protoplasm, when unenclosed in a cell-wall, can move about, direct its course, and change its shape. Knowledge of this fact did more than rectify the definition of the cell; it effaced one distinction between plants and animals, and gave a hint of the resemblance of primitive cells to such simple organisms as Amœba.

Martin Barry in 1843 announced that certain Protozoa (that name was not yet in use) are simple cells. He pointed out that they possess nuclei, like those of tissue-cells, and compared their increase by fission with the cleavage of the egg. Single cells were thus shown to be not only capable of locomotion, which was already known, but able to provide for their own support. The Protozoa and Protophyta (_i.e._, the simplest animals and plants, which are not always to be clearly separated) are now known to be autonomous cells, increasing by fission, and often forming colonies. Conjugation (fusion of similar individuals) often precedes fission, and when it was proved (1861-5) that ova and spermatozoa are true cells, it was seen that fertilisation, as we know it in the higher animals, is only a special form of the conjugation observed among the Protozoa. To the Protozoa it is now possible to trace, without any startling break of continuity, all the multicellular organisms, their tissues, the growth of those tissues by repeated fission, their eggs, and the process of fertilisation which precedes cleavage. The old Greek riddle, "Which came first, the fowl or the egg?" may now receive the answer: "Neither; their common starting-point is to be found in the Protozoa, which, even when adult, represent the primitive unicellular condition, to which all the higher animals revert once in every generation."

It is not without reason that biologists dwell on the unifying influence of the cell-theory, which has become a chief support of that still wider unifying influence, the _Origin of Species by Natural Selection_. When it was discovered that all living things, whether plants or animals, consist of nucleated cells which increase by fission, and that in all of them cell-fission is started anew from time to time by a cell-fusion, it was strongly suggested that resemblances so striking and so universal can only proceed from a common descent.

During the last half-century the study of cells has led to a great increase of knowledge respecting all bodily functions, whether in health or disease. We now look to it as perhaps the most hopeful source of new light upon the important question of hereditary transmission.

The Scientific Investigation of the Higher Cryptogams.

We now resume the history of a study which down to the end of the eighteenth century had yielded only meagre and uncertain results (see above, pp. 85-88). At the date in question it had been ascertained that the spores (then called "seeds") of ferns, and probably of other cryptogams, are capable of propagating the species, but no one knew precisely what part the spore played in the life-history, or could explain the true difference between a cryptogam and a flowering plant. The great improvements in the construction of the compound microscope which were effected between 1812 and 1830 rendered it possible to elucidate much more thoroughly the structure and development of the chief groups of cryptogams. The sexual reproduction of algæ was explored; moving filaments (spermatozoids) were seen to enter the chambers in which embryos afterwards formed; the conjugation of similar cells was observed in algæ and fungi, and recognised as a simple mode of sexual reproduction. The resemblance of the spermatozoids of mosses and ferns to animal spermatozoa was noted, and their participation in the process of fertilisation was more and more closely followed until at length Hofmeister in 1851 saw them fuse with the egg-cell of a fern. Suminski, whose full name, Lesczyc-Suminski, is unpronounceable by Englishmen, had discovered (in 1848) that the prothallus of a fern, which is the product of the germinated spore and had been hitherto taken for the cotyledon, bears two kinds of reproductive organs, one of which liberates spermatozoids, while an egg-cell is developed within the other. He did not correctly describe all the details, but he showed where the essential reproductive organs form, and where fertilisation is effected. The masterly researches of Hofmeister (1849-57) fused what had been a number of partial discoveries into a connected and luminous doctrine. He proved that the prothallus is one of two generations in the life-history; that it begins with a spore and ends with a fertilised egg-cell; that in the higher cryptogams there is a regular alternation of generations; that the prothallus of the fern answers to the leafy moss, while the leafy fern is the equivalent of the moss-capsule; that the egg-cell is the same structure in both cryptogams and flowering plants; that the pollen-tube and the seed are found to-day only in flowering plants; that the gymnosperms make a transition from the higher cryptogams to the angiosperms; that unity of plan pervades the whole series of mosses, ferns, fern-like plants, gymnosperms, and angiosperms. Before Darwin's _Origin of Species_ had appeared Hofmeister presented to evolutionists a clear example of a descent in which every principal term is well authenticated, while the extremes are far apart.

The Enrichment of English Gardens.

If some unreasonably patriotic Englishman should be seized with the whim of keeping none but truly British plants in his garden, he might enjoy the shade of the fir, yew, oak, ash, wych-elm, beech, aspen-poplar, hazel, rowan-tree, and the small willows, but he would have to forego the common elm, the larger poplars and willows, the larches, spruces, and cypresses, the rhododendrons, and all the shrubs popularly called laurels. Of fruits he might have the crab-apple, sloe, wild cherry, gooseberry, currants (black and red), the raspberry, strawberry, and blackberry, but none of the improved apples, pears, or plums, and no quinces, peaches, or apricots. His vegetable garden might yield cabbages, turnips, carrots, and celery (all deficient in size, flavour, and variety), but no cauliflowers, Brussels sprouts, parsley, lettuces, peas, beans, leeks, onions, or spinach. The handsomest of his flowers would be dog-roses, mallows, and primroses.

Before Europe was sufficiently enlightened to care about exact records valuable foreign plants had already been introduced. Vines, apples, pears, cherries, and plums, besides improved vegetables, such as the cauliflower, bean, garden-pea, and cucumber, had been brought from temperate Asia or Egypt. Wheat and barley, neither of them native to Europe, had to some extent replaced rye and oats, which may have existed naturally in those European countries which border on Asia. Britain, while yet a Roman province, shared in these benefits, and it is believed that the common elm, besides certain fruit-trees and pot-herbs, have been continuously grown in our island through all the troubled ages which separate us from the Romano-British times. Leek, garlic, and onion are ancient acquisitions. To our Old-English forefathers garlic was the _spear-leek_, distinguished by its long, narrow leaf from the broad-leaved common leek, just as a garfish was distinguished from other fishes by its long body and pointed head; onion was the _enne-_ or _ynne-leek_ (onion-leek); the most important of the three was probably that which retained the root-word without prefix—the leek proper.

During many centuries, when the rights of small proprietors were little respected and knowledge was scanty, the religious houses were distinguished by the diligence with which they tended their gardens. Flowers, fruits, and simples were cultivated, and plants were now and then imported from foreign monasteries. The English names of the plants, which are often adaptations of Latin words, still testify to the care of gardeners who were in the habit of using Latin.

Much improvement was not to be expected so long as England suffered from frequent and desolating wars within her own borders. When these at last subsided, great English gardens, such as those of Nonsuch, Hatfield, Theobalds, and Hampton Court, began to parade their beauty; strange trees, shrubs, and flowers were brought from the continent, and as early as Queen Elizabeth's time our shrubberies and walks were admired by spectators familiar with the best that Italy and France could show. The new horticulture was, however, long an exotic among us, and John Evelyn, whose _Sylva_ appeared in 1664, was "the first to teach gardening to speak proper English."

In the latter part of the sixteenth century the following new plants among others were brought from central or southern Europe: The poppy and star anemones, the hepatica, the common garden larkspur, the winter aconite, the sweet-William, the laburnum, Rosa centifolia (of eastern origin, the parent of countless varieties and hybrids), the myrtle, the lavender, the cyclamen, the auricula, Iris germanica, and many other Irids, the oriental hyacinth, several species of Narcissus, the white and Martagon lilies, and the absurdly named dog's-tooth-violet (really a lily). The botanist Clusius introduced the jonquil and the Tazetta narcissus from Spain to the Low Countries. The Judas-tree (_i.e._, tree of Judæa) was brought from the Mediterranean, where the hollows of the hills are filled in April with its pale-purple blooms. The white jasmine was imported from Asia, and the castor-oil plant from Africa.

The great accessions of geographical knowledge made during the fifteenth and sixteenth centuries were slow to affect horticulture. Ships were then few and small, and the passage from Hispaniola or Calicut to Cadiz or Lisbon occupied weeks or even months. Moreover, the conquests of Spain and Portugal (Goa, the Moluccas, Brazil, the West Indies, Peru, and Mexico) lay mostly within the tropics, and could furnish hardly any plants capable of enduring a European winter. Special pains were, however, taken to bring over some valuable food-plants which were thought likely to thrive in Europe. Before any European landed in America the potato had been cultivated by the Indians of Peru, a country which, though lying almost under the line, rises into cool mountain-districts. Potato-tubers were soon introduced to Spain and Italy, and a little later to other parts of Europe; Raleigh's planting of potatoes on his estate near Cork came a few years later. The edible tomato, which is distinguished from the wild form by its enlarged fruits, was apparently cultivated in Peru before the first landing of the Spaniards. The unusually high proportion of edible plants among the first importations from America and other distant countries is worthy of remark. Early explorers eagerly sought for valuable food-plants, but the number of such as could be cultivated alive in Europe was very limited, and since the sixteenth century the attention of collectors has been fixed upon ornamental species simply because of the dearth of others.

European flower-gardens were enriched during the sixteenth century by the following American species: the so-called French and African marigolds (both from Mexico), sunflowers, the arbor-vitæ (Thuja occidentalis), Yucca gloriosa, and the Agave, misnamed the American Aloe.

About the same time the horse-chestnut, lilac, and syringa, or mock-orange, were first brought to central and western Europe, and with them the tulip, richest and most varied of flowering bulbs. All these reached Vienna from Constantinople, but how and when they were brought to Constantinople, or what were their native countries, are still doubtful questions. The horse-chestnut is believed to be a native of Greece, where it is said to grow wild among the mountains; probably it extends into temperate Asia as well. It is said to have reached Constantinople in 1557. Longstanding tradition derives the lilac from Persia, but botanists say that it is also indigenous to parts of south-eastern Europe. The garden-tulip is believed to be native to temperate Asia and also to Thrace; it is, of course specifically distinct from the wild tulip of northern Europe.

Chief among the travellers to whom we owe the acquisition of these favourite plants was Augier Ghislen de Busbecq, a Fleming, who was twice sent by the emperor as ambassador to the sultan. Busbecq was a keen observer and collector, and during his long and toilsome journeys was ever eager to pick up curiosities or to note new facts. Quackelbeen, a physician in Busbecq's suite, is named as another helper. The botanists Mattioli and Clusius, who presided in succession over the imperial gardens of Vienna, and Gesner of Zurich, described the plants; it is from them that we draw such imperfect knowledge as we possess of the way in which they were brought to central Europe. Clusius relates that Busbecq in 1575 received a parcel of tulip-seed from Constantinople, and being obliged to journey into France, left it with Clusius to be germinated. The tulips which came up were of various colours, an indication of long cultivation. The Turks, like the Persians, took great delight in gardens.

As North America became permanently occupied by the English, facilities for the transport of live plants to Europe steadily increased. Ships began to sail frequently to and fro, for the crossing of the Atlantic was but a small affair in comparison with the voyage round the Cape of Good Hope. Educated men here and there practised the learned professions in the American plantations, and among them a sprinkling of naturalists was found. Hothouses, the amusement of wealthy amateurs in Germany, France, and Holland, made it possible to protect the plants of mild climates from the winter cold of northern Europe. By the end of the seventeenth century our gardens had acquired many beautiful and curious American plants, besides a few from the East Indies, and not long afterwards the gains became so frequent that the botanists of Europe found it hard to name the new species as fast as they came in.

Lovers of horticulture will tolerate a little further information concerning the early use of hothouses. As soon as glass began to be employed in domestic architecture, the construction of warmed and glazed chambers, in which plants could be grown, was attempted. Writers of the first century A.D. mention them, and Seneca explains how the temperature might be kept up by hot water. This and other refinements of the Roman Empire passed into oblivion during the long decline of civilisation, but revived with the revival of the arts. In the sixteenth century William IV., Landgraf of Hesse, who is remembered, among other things, as a patron of the botanist Clusius, built himself a green-house, which could be taken down and put together again. A still more famous orangerie was that of Heidelberg, which served as an example to the kings and nobles of Europe.[39] Henri IV. built one at the Tuileries, and long afterwards Louis XIV. had one at Versailles. Madame de Sévigné describes the orangerie of Clagny as a palace of Armida, and the most enchanting novelty in the world. The pine-apple was brought over from Barbadoes in the seventeenth century, and Evelyn speaks of having tasted the first pine-apple grown in England at the table of Charles the Second. For two hundred years the hothouse yielded no greater dainty, but rapid transit has now made pine-apples so cheap that it is no longer worth while to raise them in England. Fagon, who was during many years first physician to Louis XIV., was a considerable botanist. He was born and died at the Jardin des Plantes, and here, on his retirement from practice, he built hothouses; it would be interesting to know what he grew in them.

In the first half of the seventeenth century the younger Tradescant, who, like his father before him, was gardener to our Charles I., brought over from America the spider-wort, named Tradescantia after him,[40] the false acacia and the tulip-tree. The magnolias, or some of them, the Virginian creeper, and the scarlet Lobelia cardinalis were among the gifts received from North America about the same time. The dwarf Lobelia (L. Erinus) was not brought over from the Cape of Good Hope till 1752, and Lobelia splendens and fulgens (both from Mexico) not till the nineteenth century. One of the passion-flowers, which are all American, came over about this time; but Passiflora cærulea, the favourite ornament of the greenhouse, was only imported from Brazil in 1699. The evening primrose, the "convolvulus major and minor" (Ipomæa purpurea and Convolvulus tricolor), were other acquisitions from North America.

From the second half of the seventeenth century dates the introduction of the garden nasturtium (Tropæolum majus) from Peru; T. minus from Mexico had been brought over nearly a hundred years earlier. The sensitive plants and the pine-apple now became frequent objects in English greenhouses. John Evelyn and Bishop Compton were eminent patrons of English horticulture during this age.

The first half of the eighteenth century brought us the Aubretia and the sweet pea from southern Europe, the first Pelargoniums (scarlet geraniums) from the Cape, the camellia and Kerria japonica from the far east. The West Indian heliotrope was introduced in 1713; the better-known Peruvian species not till 1757. Phloxes began to be imported from North America. Two or three foreign orchids were already known, and the number now began to increase; but it was not till the nineteenth century that they came over in crowds. Our list gives no notion whatever of the number of new species added now and subsequently.

Of the accessions made during the latter half of the eighteenth century we must at least mention the mignonette from North Africa, white arabis from the Caucasus, the common rhododendron from Asia Minor, Rosa indica and Hydrangea hortensis from China, South African gladioli, which now begin to be numerous, and chrysanthemums from China and Japan. The first calceolarias were brought from great heights on the Andes, the first begonias from Jamaica, and the first fuchsia from Chili.

We can make only one remark about the multitudinous accessions of the nineteenth century. It is surprising to note how recently many established favourites have been brought to the knowledge of English gardeners. Anemone japonica (Japan) and Jasminum nudiflorum (China) date from 1844, while the Freesias (Cape Colony) are as recent as 1875. The dahlia, after two unsuccessful attempts, was established here as recently as 1815; Nemophila insignis came over from North America in 1822; the common musk and the monkey-plant a few years later; the chionodoxas (Crete and Asia Minor) in 1877. The first of the foliage-begonias (Begonia rex from Assam) dates only from 1858, and the first of the tuberous species from 1865.

Importation of foreign species has not been the only method by which English gardens have been enriched. New varieties and hybrids have been produced in bewildering numbers by the gardeners of Europe, and many of these far surpass in beauty the wild originals. Botanists and nurserymen could relate in great detail the steps by which our favourite roses, calceolarias, begonias, and cinerarias have been developed from a few natural stocks, sometimes of uninviting appearance.

Horticulture has repaid the debt which it owed to the explorations of botanists by furnishing countless observations and experiments bearing upon inheritance. When these have been properly co-ordinated, they will yield precious knowledge, not only to botanists but to all students of biology.

Humboldt as a Traveller and a Biologist.

The career of Alexander von Humboldt (b. 1769, d. 1859), nearly coinciding with the period on which we are now engaged, was devoted to a gigantic task—nothing less than the scientific exploration of the globe. His great natural powers were first cultivated by wide and thorough training, not only in astronomy, botany, geology, mineralogy, and mining, which had an obvious bearing on his future enterprise, but also in anatomy, physiology, commerce, finance, diplomacy, and languages. Thus equipped, he sailed in 1799 with the botanist Bonpland to South America, and spent the next five years in exploring the Orinoco and Amazon, the Andes, Cuba, and Mexico. The expedition marks an epoch in scientific geography. It is enough to mention the collection of data for the more accurate mapping of little-known countries, the exploration of the river-systems of equatorial America and the discovery of a water-connection between the Orinoco and the Amazon, the ascent of lofty mountains, the study of volcanoes, the description of remarkable animals such as the howler-monkey and the gymnotus (electric eel), and of remarkable plants, such as the bull's-horn acacia, whose enlarged and hollow spines are occupied by ants.[41] After his return to Europe Humboldt published many important treatises on terrestrial magnetism, geology, meteorology, and plant-distribution. His new graphical method of isothermal lines did much for the study of climate in all its bearings. His _Personal Narrative_ not only disseminated much interesting information, but inspired a new generation of explorers. Darwin agreed with Hooker that Humboldt was the greatest of scientific travellers.

In 1829 Humboldt traversed the Russian Empire from west to east, but the time allowed (half a year) was altogether insufficient for the examination of so vast a territory; a few notable results were, nevertheless, secured.

After some fifteen or twenty years spent in European society, the inspiration drawn from long and arduous journeys in South America began to fail. The conversation of the _salons_, the troublesome flattery of the King of Prussia, and the propensity to write copiously, stimulated, of course, by the eagerness of the public to buy whatever so eminent an investigator chose to put forth, sterilised the last half of a career which had opened with such magnificent promise.

The best of Humboldt's work became absorbed long ago into the confused mass of general knowledge. This is the common fate (not by any means an unhappy one) of those who refuse to concentrate upon a single study. Among biologists he is chiefly remembered by his numerous discussions of plant-distribution, which are now considered less remarkable for what they contain than for what they leave out. While his travels were fresh in his mind, Humboldt was impressed by facts of distribution which could not be explained by present physical conditions,[42] but the influence of climate as the more intelligible factor gradually assumed larger and larger proportions in his mind. The writers of text-books, founding their teaching upon Humboldt, often overlooked altogether qualifications which he had shown to be necessary. When Darwin and Wallace pointed out how immensely important is the bearing upon present distribution, not only of the physical history of the great continents, but also of their biological history, and in particular of the interminable conflicts of races of which they have been the scene, naturalists began to perceive how inadequate are horizontal and vertical isothermal zones to explain all the striking facts of distribution, whether of plants or animals (see _infra_ p 129).

Premonitions of Biological Evolution.

The eighteenth century had done much to impress the minds of men with an orderly development in sun and planets (Kant and Laplace), in the institutions of human societies (Montesquieu), and in the moral aspirations of mankind (Lessing). Many bold attempts had been made to trace a like orderly development in the physical life of plants and animals (Buffon, Erasmus Darwin, etc.), but neither was the proof cogent nor the process intelligible. Cautious people therefore, and those whose prepossessions inclined them to adopt a very different origin for terrestrial life, held during all this time a position of some strength against speculative philosophers who tried to explain the variety and perfection of living nature by unconscious and unintelligent factors.

About the year 1840 the doctrine of the fixity of species seemed to be victorious. Cuvier's knowledge and skilful advocacy had a few years before over-powered Geoffroy St. Hilaire's conception of a common plan of structure pervading the whole animal kingdom, and the new _Philosophie Anatomique_ was laid on the shelf, side by side with the _Philosophie Zoologique_ of Lamarck, the _Zoonomia_ of Erasmus Darwin, the _Théorie de la Terre_ of Buffon, and the _Protogæa_ of Leibnitz. Yet even then a spectator who was fully informed and at the same time gifted with uncommon foresight might have satisfied himself that the victory of evolution had become inevitable.

Cuvier's memorable descriptions of the extinct vertebrates of the Paris basin had founded the new science of Palæontology, and though neither he nor anyone else was aware of the fact, had made it possible to trace, very imperfectly no doubt, the descent of a few modern ungulates. Lyell's _Principles of Geology_ (1830-3) had shaken the belief in catastrophes repeatedly breaking the succession of life on the earth. It was rapidly becoming impossible to maintain that the account of creation given in the book of Genesis was even approximately accurate. In the year 1828 Baer had almost made up his mind that the facts of development pointed to a common plan of structure, perhaps to a common origin, for each of the great types of animal life.[43] Darwin's _Journal_ had appeared in 1839, and though the explanations which it offered were not inconsistent with prevalent opinion, evolutionary suggestions were introduced into the second edition of 1845. Lyell at least was already aware that the voyage of the _Beagle_ had impelled Darwin to examine afresh the accepted philosophy of creation. Between 1840 and 1850 faint signs of coming change struck orthodox reasoners with misgiving and gave increased confidence to free-thinkers. A few German botanists and zoologists declared against the immutability of species. The _Vestiges of the Natural History of Creation_, which might be called a premature explosion, dates from 1844. Hofmeister (see supra, p. 109) put forth a detailed comparison of the flowering plants with the higher cryptogams, which strongly suggested a theory of descent with modification, and is unintelligible on any other basis. He indicated no such interpretation himself, being content to establish the new homologies; but the _Origin of Species_, as soon as it appeared, commanded his entire sympathy.

Among those who rejected fixity of species and special creation before 1859 none was so clear or so outspoken as Herbert Spencer, who thought out for himself an evolutionary philosophy which was not shaken by Darwin. It is impossible to discuss in this place the question whether or not it was shaken by Weismann.

Agassiz's _Essay on Classification_, which was published in October, 1857, was the last manifesto issued before the _Origin of Species_ by the party which stood out for fixity of species, the last polemic which made De Maillet, Lamarck, and the _Vestiges_ its targets. It is an eloquent but inconsiderate defence of an extreme position. According to Agassiz every branch, class, order, family, genus, and species represents a distinct creative thought; every mark of affinity, every appearance of adaptation to surroundings, has been expressly designed. Extinction and replacement of species are due to the direct intervention of the Creator; pterodactyls are prophetic types of birds, and indicate that divine wisdom had foreseen the possibility of an advance in the organisation of animals which was not immediately practicable; the mallard and scaup duck occur on both sides of the Atlantic because they were simultaneously but separately created in Europe and North America; the teeth of the whale, which never cut the gum, are the result of obedience to a certain uniformity of fundamental structure. Explanations like these removed no difficulties and suggested no inquiries. In the hot debates which ensued the _Essay on Classification_ was rarely mentioned.

[32] _Cross and Self-Fertilisation of Plants_, chap. xi. Darwin could exhaust the inquiry. "The veil of secrecy," he goes on, "is as yet far from lifted."

[33] Cuvier did not himself use the word _palæontology_, which first came in about 1830. In the same way Buffon writes on the history of animals, not on _zoology_, and on the theory of the earth, not on _geology_.

[34] This anecdote has also been related in a rather different form.

[35] The same process of "embryonic fission" occurs in other animals also, one of which is a mammal (Praopus).

[36] Linnæus (Fund. Bot. § 134, and Sponsalia Plantarum) gives it as above; Harvey has "Ex ovo omnia"; "ovum esse primordium commune omnibus animalibus," etc.

[37] Harvey need not have gone outside the writings of Aristotle to get the substance of his generalisation. He would have found there that the chief task of both plants and animals is propagation, either by seeds, or eggs, which Aristotle believed to be equivalent to seeds (_Hist. anim._, VIII., i.; _De anim. gen._, I., iv.; I., xxiii.). Aristotle excepted the "imperfect animals," such as insects, and the seedless plants, concerning both of which his knowledge was misty and inaccurate; there is no indication that Harvey was better informed.

[38] Hooke figured a thin section of dry cork in his _Micrographia_ (1665), remarking that it was divided into "little boxes or cells." The word _cell_ was suggested by the resemblance of the tissue to a honeycomb; since 1838 it has been thoughtlessly extended from the skeleton to the particle of living matter enclosed within it. Robert Brown (1831) showed that a nucleus is usual in plant-cells; it had been figured by Fontana and others long before. Down to 1838 no results of biological interest followed from the discovery.

[39] Parkinson (1629) speaks of a stove or hothouse, "such as are used in Germany."

[40] The graceful practice of naming genera of plants after benefactors to botany or horticulture was introduced by Father Plumier (1646-1704), who gave the names of L'Obel and Fuchs to the Lobelia and Fuchsia, and whose own name is appropriately borne by the frangipane (Plumeria).

[41] See the account of Cartagena in the _Personal Narrative_.

[42] See particularly his _Essai sur la géographie des plantes_ (1805).

[43] Baer's expressions are so guarded that his real opinions in 1828 can only be surmised. He never accepted a consistent theory of organic evolution.

PERIOD V.

(1859 AND LATER)

Period V.

We do not attempt to characterise our last period, nor to describe its biological achievement. It seems better to devote the whole of our scanty space to the scientific careers of Darwin and Pasteur, in which so much past effort culminated, and from which so much progress was to spring.

Darwin on the Origin of Species.

Setting aside as superfluous and we might say impossible, under our conditions of space, all attempt to restate the evidence on which Darwin based his great argument, we shall here try to show that the _Origin of Species_ shed a new light upon many biological facts, combined many partial truths into one consistent theory, and gave a great stimulus to further inquiry.

1. _Classification and Affinity._—The sixteenth-century herbalists and still earlier writers (see p. 17) recognised a property of _affinity_, by which plants were associated in natural groups. Bock ( 1546) tried to bring together all plants which are related (verwandt) to one another, but similarity of any kind was with him a proof of affinity; it did not shock him to place the dead nettles next to the stinging nettles. L'Obel gave names to several families of flowering plants which are still admitted as natural. Ray spoke of the _affinity_ (cognatio) between plants, and his affinity was a thing not to be violated for the sake of practical convenience or logical rules, but he was unable to explain what he meant by it. Linnæus tried to illustrate affinity between plants by contiguous provinces on a map, a better metaphor than the linear scale, for the scale can only express affinity on two sides, while the map can express affinity on many. His practical experience of classification taught him a truth, shocking at first sight to the logician[44]—viz., that the characters which serve for the definition of one genus may be useless for the definition of the next, and he laid it down that the characters do not make the genus, but the genus the characters. After Linnæus we find for a long time no advance in the philosophy of natural classification. Cuvier (1816) is even retrograde, for he sets aside the maxims of Linnæus, maintains that adaptive characters (characters closely related to the conditions of life) are relatively constant, and that large groups should be defined by characters drawn from organs of great physiological importance. These decisions of his are repudiated by later naturalists.

The key to the affinity puzzle which had so long baffled thinking naturalists was at last supplied by Darwin, who explained that "the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, all true classification being genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike."[45]

Natural groups, large or small, result from the long-continued operation of divergence, the survival of some, and the extinction of others; they are to be respected as facts; they are not created by definitions, which only serve to indicate and remind; any character, however trifling, will suffice, if only it is constant and distinctive.

The conflict between natural classification and logic is apparent only. Logicians say that in classifying books, for instance, you may take any property you please, subject, size, etc., as the basis of your arrangement, but having made your choice, you must adhere to it for all divisions of the same rank. Naturalists seem to say something different, for they are agreed that what they call "single-character classifications," in which one property is adhered to throughout, are unnatural. The fact is that a natural classification always rests upon one and the same property—viz. _affinity_, _i.e._ relative nearness of descent from some common ancestor. Every natural classification, like every logical classification, proceeds upon a single basis, and the failure of the single-character classifications is due to their displacing affinity by some definition.

The effect of the _Origin of Species_ upon zoological and botanical systems has been revolutionary. Furnished with a new and intelligible meaning of the word natural, and with new criteria of naturalness, systematists have during the last fifty years worked hard to create classifications which admit of being thrown into the form of genealogical trees. Wide gaps in the geological history of life render the task difficult beyond expression, but much has already been accomplished. Newly discovered forms (especially the fossil Archæopteryx and the Cycadofilices) and more fully investigated forms, far too numerous to be specified, have established links between groups which formerly seemed to be wholly independent. Unnatural assemblages based on pre-determined characters (Radiates, Entozoa, Birds of prey, etc.) have been replaced by groups which are at least possible on evolutionary principles. Almost every working naturalist will admit that the progress of zoological and botanical system during the last two generations has done much to fortify the Darwinian position.

2. _Embryology._—Baer in 1828 was possessed of all the embryological facts which Darwin used in support of his theory of evolution; in particular, he was well acquainted with the most striking fact of all—viz., the presence in embryo mammals and birds of a series of paired clefts along the sides of the neck, between which run vessels arranged as in gill-breathing vertebrates. The vessels had been figured by Malpighi; the clefts had been discovered by Rathke, who had no hesitation in calling them _gill-clefts_ and the vessels _gill-arches_. Nor had Baer, who nevertheless to the end of his long life refused to accept the one explanation which gives meaning to the facts—viz., that remote progenitors of mammals and birds breathed by gills. Few embryologists have since felt such a scruple. The adaptation to gill-breathing is obvious; is gill-breathing _now_ practised by any mammal or bird? Certainly not. Is it destined to be practised by their descendants at some _future_ time? To say nothing of the danger of putting forth any such prophecy, it involves all the consequences of descent with modification. The opponent of evolution may as well admit at once that the gill-breathing was practised in time _past_. As an example of the same kind taken from plants, we may quote the trifoliate leaves of the furze-seedling, which, though absent from the full-grown furze, are frequent in the family (Leguminosæ) to which it belongs. The general similarity of vertebrate embryos, of insect-embryos and of dicotyledonous seedlings, is also worthy of note. We may suppose that early embryos, being largely or wholly dependent on food supplied by the parent, and perhaps protected by the parent as well, escape the pressure of the struggle for existence, and are often not urgently impelled to produce adaptations of their own. In these circumstances it is intelligible that features inherited from remote ancestors should persist. If, however, early independence is demanded by the conditions of life, the embryo may develop temporary adaptations, wanting in the parent and in embryos of allied groups. Larval adaptation is as much a part of the economy of nature as the retention of ancestral structures which have been lost by the adult.

3. _Morphology._—Let us next consider the light which the Origin of Species throws upon homologous parts. No example will serve our purpose better than the very familiar one of the fore-limbs of different vertebrates, the arm and hand of man, the wing of the bat, the wing of the bird, the pectoral fin of the fish, and the paddle of the whale. These limbs, adapted for actions so diverse as grasping, running, flying, and swimming, nevertheless exhibit a common plan, evident at a glance, except in the pectoral fin of the fish. But why a common plan? Of what advantage is it to an animal that its wing, paddle, or hand should reproduce the general plan of a fore-foot? Why should the digits of the land vertebrates never exceed five? Why should the thumb never have more than two free joints? It would be hard to find a satisfactory answer to these questions in any book earlier than the _Origin of Species_; no student of the _Origin of Species_ finds any difficulty in answering them all. The common plan has been transmitted from type to type by inheritance, and its features are derived from an unknown common ancestor.

The new conception, that structures inherited from remote ancestors may be incessantly modified by the conditions of life and by mutual competition, is the key to the chief problems of morphology. No limited collection of examples can substantiate so wide a proposition as this. Those who have made themselves familiar with old text-books of comparative anatomy will recollect how dry, or else how inconclusive, was pre-evolutionary morphology, how vague were the references to some ideal archetype, or to climate, or to the ancient conditions of the earth's surface; how often exclamations of admiration for the marvels of nature or Providence were substituted for clear explanations. Cuvier, it is true, was both precise and reasonable; but how little he was in a position to explain! His "empirical" comparative anatomy could throw no direct light upon origins or transformations; his "rational" comparative anatomy was practicable only in a few easy cases.

4. _Geographical Distribution._—The facts of distribution were handled in the _Origin of Species_ with great originality. It was shown that they support, and indeed require, some doctrine of organic evolution. The succession in the same area of the same types—armadillos succeeding armadillos in South America, marsupials succeeding marsupials in Australia—was enough of itself to render independent creation highly improbable. This was not all. Darwin's mind being charged with facts and reasonings, the accumulations of many years of travel and meditation, he sketched in rapid outline conclusions which have given a new form to the distribution question. The subject had hitherto been treated by collecting masses of facts and interpreting them by recent physical geography; Darwin showed that the _history_ of the continents and islands may be far more influential than soil, elevation, or climate.

The scientific discussion of the facts of distribution is as old as the sixteenth century, when L'Obel pointed out that the mountain plants of warm countries descend to low levels in the north. Linnæus remarked that fresh-water plants and alpine plants are often cosmopolitan. Another early and well-founded generalisation is the statement of Linnæus that the plants common to the old and the new world are all of northern range. Buffon made the same remark about the animals, and offered the probable explanation—viz., that since the two great land-masses approach one another only in high latitudes, it is only there that animals have been able to cross from one to the other.

In the nineteenth century theories involving prodigious changes of land and sea were much in the minds of naturalists. Darwin lost his temper (a rare thing with him) over the land-bridges, hundreds, or even thousands, of miles long, which were created in order to explain trifling correspondences in the population of distant countries. A belief in the comparative stability of the great continents and oceans has since prevailed, and it is now recognised that the means of dispersal of species are greater than was once supposed.

The discovery, about the year 1846, of the marks of ancient glaciers in all parts of northern Europe, and the acceptance of an Ice Age, had a still greater influence upon the teaching of naturalists. Edward Forbes[46] put forth a glacial theory to account for the present distribution of plants of northern origin. Glacial cold, he maintained, had driven the arctic flora far southward. When more genial conditions returned, most of the northern plants retreated towards the Pole, but some climbed the mountains and gave rise to an isolated alpine flora. Darwin, whose unpublished manuscripts had anticipated Forbes's theory, believed that the whole earth became chilled during the Ice Age, and that the fauna and flora of the temperate zone reached the tropics. His argument, which is contained in chap. xi. of the _Origin of Species_, is now generally accepted in principle, though opinions differ on many points of detail. Some think that he extended too widely the effects of glacial cold, exaggerated the resemblance of the arctic and alpine fauna and flora, and attributed the extinction of the northern species in the intermediate plains too exclusively to climatic causes.

One paragraph in the extremely condensed discussion on geographical distribution which we find in the _Origin of Species_ calls attention to the dominance of forms of life "generated in the larger areas and more efficient workshops[47] of the north." The power which inhabitants of the great northern land-mass of the old world, and in a less degree those of North America, possess, and have long possessed, of driving out the inhabitants of the southern continents is one of the most important factors in the peopling of the earth with new races of land-plants and land-animals. Races of men, modes of civilisation, religious faiths, all follow the same rule, which has no doubt prevailed ever since land came to predominate in the northern hemisphere and water in the southern hemisphere. In the life of the sea and the fresh waters no dominance of northern forms has been detected.

5. _Palæontology._—We must not claim for Darwin more than a modest share in the vast extension of palæontological knowledge which the last fifty years have created. A profusion of new materials has been acquired by the diligence of collectors working on a scale previously unattempted. But though the accumulation of materials is the work of others, the interpretation has been guided by the principles of Darwin. The evolution of the horse has now been so fully worked out that it would bear the whole weight of a doctrine of descent with modification, though it could not by itself reveal the process by which the modification had been effected.

_Darwin on Adaptations._—The adaptation of living things to their surroundings has always been a favourite branch of natural history, underrated only by those whose studies are little calculated to inflame the curiosity. Many eminent naturalists have made the interpretation of natural contrivances their chief aim. Darwin equalled the best of his predecessors in accuracy, range, and ingenuity, while he surpassed them all in candour. No one has done so much to vindicate the study of adaptations from all suspicion of triviality, for no one before him had seen so clearly how all new species arise by adaptation of pre-existing ones. It is by adaptation that new forms of life arise; it is inheritance which preserves old ones.

Socrates, Swammerdam, and Paley had drawn from the adaptations of nature proofs of the omnipotence and beneficence of the Creator. Darwin, while admitting that every organism is exquisitely adapted to its own mode of life, believed that the adaptations have been perfected by slow degrees, and that they cannot be proved to have been consciously devised. This interpretation deprives the theologian of valued arguments, but at the same time rids him of difficulties. Even before Darwin's day some few natural theologians had the courage to bring forward instances of the harshness of nature. Kirby and Spence[48] thought that no injustice was done to certain predatory insects by comparing them to devils. Others blessed the mercy of heaven, which, after creating noxious animals, created others to keep them in check. Darwin, when reflecting upon the odious instincts which urge the young cuckoo to eject its foster-brothers, some species of ants to enslave others, and a multitude of ichneumons to lay their eggs in the bodies of live caterpillars, found it a relief to be able to shift the responsibility to an unconscious natural process.[49]

In his autobiography Darwin remarks that he had thought it almost useless to endeavour to prove by indirect evidence that species had been modified until he was able to show how the adaptations could be explained. Some of them alarmed him by their difficulty; to suppose that the eye, with all its inimitable adjustments, had been formed by an unconscious natural process seemed to him absurd until he had traced a good many intermediate steps between the mere colour-spot and the eye of the eagle. He writes to Asa Gray (September 5, 1857) that the facts which had done most to keep him scientifically orthodox were facts of adaptation, the pollen-masses of Asclepias, the mistletoe with its pollen carried by insects and its seeds by birds, the woodpecker exquisitely fitted by feet, tail, beak, and tongue to climb trees and capture insects.

The student of adaptations has no longer a moral thesis to maintain; he tries to understand how a contrivance acts, what advantage it confers upon its possessor, and by what steps it was perfected. The minute variations of species are as capricious as the form of the stones which accumulate at the foot of a precipice; natural selection turns fortuitous variations to account for the advantage of the species as a builder might turn to account the shapes of the stones. Man himself can employ variations for frivolous or even base purposes, as when he produces toy-spaniels or bull-dogs.[50] The adjustments of organic structures often move our wonder by their perfection. One reason why they so far exceed the adjustments made by wind, frost, or moving water is that the process has been so protracted; in a worm or an insect we see the last stage of an adaptation which has been continuously at work for untold geological periods. Another reason is that the thing adapted is alive, sensitive, and capable of responding to the subtlest imaginable influences.

_Darwinism and Non-biological Studies._—The theory of organic evolution has already produced a visible effect upon non-biological studies. Bagehot has applied Darwinian principles to the interpretation of history and politics. Philologists recognise a process very like that of natural selection in the modification of words. The usages of language are inherited from generation to generation; one idiom competes with another, that persisting which best suits the temper or the convenience of the nation. Philology has like zoology its chains of descent, its breeds or dialects, its species or languages, its fossils (dead languages), its dominant and declining forms, its vestiges (such as letters, still retained, though no longer sounded). Psychology is already in part experimental and evolutionary, and seems as if it would attach itself more and more closely to physiology, detaching itself in the same measure from metaphysics. The change may be attributed to two growing convictions: (1) That the experimental method is more trustworthy than the speculative; and (2) that the mind of man is not a thing apart, but an enhanced form of powers manifest in the lower animals. Sociology finds its most practicable and its most urgent sphere of work in the problems of selection and race, which are naturally examined in the light of Darwinian principles. The new study of Comparative Religion aims at the impartial examination of all forms of religious experience, and is evolutionary in proportion as it is scientific. One of its conclusions, by no means universally accepted as yet, is the recognition of conscience as "the organised result of the social experiences of many generations" (Galton). Comparative Religion can already show in outline how by slow degrees magical rites passed into polytheistic worship, how polytheism became simplified and elevated, and how ethical motives at length became influential if not predominant.

Pasteur's Experimental Study of Microbes.

The same difficulty arises with Pasteur as with Darwin; his life-work has already been described often and well. Readers unversed in science have only to turn to the _Vie de Pasteur_, written by his son-in-law, Vallery-Radot, to find a luminous account, giving just so much detail as makes the discoveries intelligible and interesting. If shorter sketches are demanded, they exist. We must therefore above all things be brief, and content ourselves with reminding the reader of facts which, in spite of their recent date, are as well known as anything in the history of science.

Chemists will claim Pasteur as one of their number, and we do not dispute the claim. Trained in experimental methods by the chemical laboratory, he devoted his best powers to the study of living things, and, without ceasing to be a chemist, became one of the greatest of biologists.

Pasteur's chief work was of course the experimental investigation of living particles which float in the air—what we may call _live dust_. Before his day such particles had been seen, named, and classified; some few had been studied in their action and effects. Most of them are plants of low grade, simplified to the last point for the sake of minuteness, on which their ready dispersal depends.

_Yeast._—Van Helmont, early in the seventeenth century, when the microscope had not yet become an instrument of research, attempted to investigate the fermentation of beer, and made acquaintance with the properties of the gas which is evolved, his _gas silvestre_, which was afterwards called fixed air, or carbonic acid. Leeuwenhoek about 1680 examined yeast by his microscopes, and discovered that it is made up of globules which often cohere, and that these globules give off bubbles of gas. Then comes a long interval, during which nothing was done to elucidate the process of fermentation. It was not till 1837 that Caignard-Latour and Schwann, independently of each other, showed that yeast-globules multiply by budding, and are therefore to be set down as living things, probably plants of a simple kind. Twenty years more passed without sensible progress; during this time chemists were striving to prove that the alcohol was produced by contact-action, and that the globules were of no practical importance. By the year 1860 Pasteur was engaged upon the problem. It is well known that he arrived at a firm conviction that living yeast-cells are essential to the production of alcohol. It has since been discovered that the enzyme (unorganised ferment of older writers) secreted by living yeast-cells can change sugar into alcohol after the cells themselves have been destroyed, and that other plants besides yeast-cells secrete the same enzyme when deprived of oxygen.