Part 12

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. 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, 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. 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.

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."

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.

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

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

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

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.

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.

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

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).

See the account of Cartagena in the Personal Narrative.

See particularly his Essai sur la géographie des plantes (1805).

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.