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CHALES DARWIN is besides GREGOR MENDEL the most important biologist of the 19th century. His most important work,
On the origin of species by means of natural selection or the preservation of favoured races in the struggle of life,
was published in 1859. It marks a turning point in the history of evolutionary research. Rudimentary and little noticed ideas about evolution and about changes of flora and fauna in the course of time existed already in antiquity (EMPEDOKLES, ARISTOTLE), but conclusive proof was missing.
R. HOOKE (1635, Isle of Wight, – 1703), one of the first microscopists, who coined the term ‘cell’, described numerous fossil structures from the English Jurrasic period and found that the structures of different geological layers differed. He avoided, nevertheless, the conclusion that an evolution could have taken place, but did assume instead that the fossils were relicts of organisms killed by the Flood. This idea was at the beginning of the 19th century taken up by the Swabian natural scientists G. CUVIER (1769 – 1832), who developed it into the theory of catastrophes, also called catastrophism. It says, that fossils are remains of subsequent series of floras and faunas that perished one after the other due to catastrophes. Though CUVIER recognized, that fossils of different layers differ, he did not observe, that younger layers do indeed contain fossils of more complex species, while the fossils of species in older layers are more primitive. He did thus not understand, that the complex species have developed from the more simple ones.
It was the Swiss physician and mathematician JOHANN JAKOB SCHEUCHZER, who saw the significance of plant fossils and who recorded his findings in his illustrated work ‘Herbarium Diluvianum’.
The French zoologist G. L. BUFFON (1707 – 1788) succeeded in a decisive breakthrough, when he pointed out in his work ‘About the common descent from ancestors’ that not only donkey and horse, but human and monkey, too, belong to a natural family. BUFFON addressed a number of evolutionary problems nobody else had realized before. He added these topics to the repertoire of scientific questions, even though his own conclusions were often wrong. In his opinion, the majority of variations was of a non-genetic nature and caused by the environment instead. This idea was taken up by his pupil JEAN BAPTISTE de LAMARCK (1744 – 1829), who strengthened it in his work ‘Philosophie Zoologique’ published in 1809.
‘Since every species has to exist in perfect harmony with its surrounding and since this surrounding is constantly changing, the species itself, too, has to change constantly, if it is to stay in a harmonic balance with its surrounding. If it would not adjust, the species would be threatened by extinction.’
LAMARCK did thus discover the time factor in the evolution of organisms. He worked on botanical topics writing a work in four volumes about the flora of France before he began to study evolutionary problems. Later on, he concentrated on invertebrates. The overall classification he suggested for invertebrates is still valid. In 1801, he coined the term biology.
In his ‘Philosophie Zoologique’ LAMARCK assumed; that the term ‘species’ is practicable and makes sense, but that species could sometimes merge into other species without clear demarcations. He concluded, that in nature, only individuals that can essentially be grouped into a step-like organisation exist. This, he found, is especially clear when descending from the more complex to the simpler structures. He illustrated ‘species formation’ in plants with the following example.
"As long as Ranunculus aquatilis grows submerged, all leaves are differentiated into small segments. But as soon as a stalk reaches the surface, large, round, and only simply lobed leaves develop. If a stolon of the same plant grows on moist soil without being submerged, then short stems grow, no leaf is strongly lobed any more, and the result is Ranunculus hederaceus classified by botanists as a species of its own."
A transformation of species the way he imagined it does not occur in nature. A simple experiment, the cultivation of Ranunculus aquatilis on moist soil would have prevented LAMARCK’s false conclusion. His work can nevertheless and despite its many unproven statements and its unanswered questions be regarded as the first clear and consequent theory of descent.
During the first half of the 19th century it became quite obvious, that the organization, behaviour and occurrence of plants and animals is shaped by the environment. It was also realized that the surface of the earth is constantly changing. The English geologist CHARLES LYELL (1797 – 1875 - picture to the left) drew a picture of the genesis of the earth largely still valid today. He explained, for example, the origin of layers by subsequent depositions and the development of rocks by pressure and plication. He found it unscientific to assume, that the past was ruled by other forces than the present is. He changed his assumption that the species are immutable, when learning about the observations and ideas of DARWIN. LYELL’s ideas had a lasting effect on DARWIN.
DARWIN was just as much influenced by the ideas of the English economist THOMAS ROBERT MALTHUS (1766 – 1834). MALTHUS published in 1798
An essay on the principle of population.
It pointed out the problem of a population explosion due to the difference between exponential growth of the human species and linear growth of food production. He foresaw the overpopulation of the earth as a result of its limited capacity, so that in the long term and without suitable control mechanisms only part of mankind would survive. MALTHUS saw clearly that the limited resources of the earth would cause a struggle for existence.
Several natural scientists of the 19th century came to similar conclusions by studying individual cases, but they did not succeed in uniting these cases in a comprehensive theory.
DARWIN cites W. C. WELLS as the discoverer of the principle of natural selection. In 1818, WELLS wrote:
"An Account of a White female, part of whose skin resembled that of a Negro'; but his paper was not published until his famous 'Two Essays upon Dew and Single Vision' appeared in 1818. In this paper he distinctly recognises the principle of natural selection, and this is the first recognition which has been indicated; but he applies it only to the races of man, and to certain characters alone. After remarking that negroes and mulattoes enjoy an immunity from certain tropical diseases, he observes, firstly, that all animals tend to vary in some degree, and, secondly, that agriculturists improve their domesticated animals by selection; and then, he adds, but what is done in this latter case ' by art, seems to be done with equal efficacy, though more slowly, by nature, in the formation of varieties of mankind, fitted for the country which they inhabit. Of the accidental varieties of man, which would occur among the first few and scattered inhabitants of the middle regions of Africa, some one would be better fitted than the others to bear the diseases of the country. This race would consequently multiply, while the others would decrease; not only from their inability to sustain the attacks of disease, but from their incapacity of contending with their more vigorous neighbours. The colour of this vigorous race I take for granted, from what has been already said, would be dark. But the same disposition to form varieties still existing, a darker and a darker race would in the course of time occur: and as the darkest would be the best fitted for the climate, this would at length become the most prevalent; if not the only race, in the particular country in which it had originated."
RAFINESQUE gives in his ‘New Flora of North America" (1836) a botanical example:
"All species might have been varieties once, and many varieties are gradually becoming species by assuming constant and peculiar characters."
C. NAUDIN wrote in 1852 that species in nature developed analogous to varieties in culture. He ascribed the latter process to the human ability to choose. How it occurs in nature was not among his interests.
In 1853, SCHAAFHAUSEN postulated in an essay (published in "Verhandlungen des Naturwissenschaftlichen Vereins der Preußischen Rheinlande") that the growing differentiation of the species had to be explained by a destruction of the intermediate stages.
"Thus living plants and animals are not separated from the extinct by new creations, but are to be regarded as their descendants through continued reproduction."
The most important of the many predecessors of DARWIN was the Viennese botanist F. UNGER (1800 – 1870) who happens to be one of the teachers of GREGOR MENDEL and A. KERNER von MARILAUN. In his paper "Versuch einer Geschichte der Pflanzenwelt" (Attempt of a history of the plant world) published in 1852, he writes:
" In this marine vegetation consisting of thallophytes, i.e. algae, the true seed of all the plant structures surfacing gradually throughout history has to be made out. It cannot be doubted that the origin of the plant kingdom traced back by experience thus far, can theoretically be traced back even further, so that at last a first plant, or moreover a cell being the cause of all plant existence would be detected. "
"... In the relations of the species, by far too many regularities exist for the assumption that the origin of new species could be caused by extern influences....This means that the reason for the development of all these differences of a plant cannot be extern but have to be intrinsic...In short, each newly developing plant species...has to come from another plant species."
In 1858, C. LYELL informed C. DARWIN about a work of A.R. WALLACE (1823 – 1913 - picture to the right) explaining WALLACE’s theory of species formation. WALLACE had studied the distribution of animal species and concluded that both a selection and an evolution had to have taken place. This conclusion was influenced by the ideas of MALTHUS. Another formative influence of WALLACE was his teacher W. BATES (1825 – 1893) whose theory of mimicry he extended further. In 1858, the essay
On the tendency of varieties to depart indefinitely from the original type
was published in the ‘Transactions of the Linnean Society’ (editor: LYELL). The same issue contained a notice of DARWIN pointing out the publication of his main work in the next year.