DARWIN studied first medicine at the University of Edinburgh and subsequently theology at the University of Cambridge, but his grandfather’s work "Zoonomia or the laws of organic life" (1794/96) containing some initial ideas about the evolution of organisms and his friendship with the botanist J. S. HENSLOW from Cambridge influenced him a far more.
From 1831 to 1836, DARWIN participated as a natural scientist in a journey around the world on board of the research ship ‘Beagle’. The journey became a crucial experience for DARWIN and was the starting point of all of his later works. During this time, he made the first observations for his theory of selection. The study of marine fossils in the higher Andes and the characteristic flora and fauna of the Galapagos Archipelago impressed him particularly.
He summarized the results of this journey in a preliminary report followed in 1860 by the detailed version
The Voyage of the Beagle
The German translation was published in 1875. Some years after his return to England, DARWIN moved to his country seat Down House in the county Kent south of London, where all his basic works were written.
After some years of research into geological problems during the forties of the 19th century, he began to work on his theory of selection. In 1859, his main work
was published followed , in 1869, by the two volumes of
"The variation of animals and plants under domestication".
Around 1860, he entered a time of his life, that was chiefly dedicated to botanical problems. He backed up many of his statements by specific experiments (see next section). He collected the results and his conclusions in a number of publications. Their titles reveal his broad field of interest and research:
1862: The various contrivances by which orchids are fertilized by insects.
1867: On the movements and habits of climbing plants.
1875: Insectivorous plants.
1876: The effects of cross- and self-fertilization in the vegetable kingdom.
1877: The different forms of flowers on plants of the same species.
1880: The power of movement in plants.
1881: The formation of vegetable mould, through the action of worms, with observations on their habits.
Shortly after their first publication, J. V. CARUS translated all publications of DARWIN into German.
DARWIN’s study Down House, Down, Kent
Under the impression of MALTHUS’ ideas, DARWIN writes at the beginning of his main work, that the fact, that altered conditions of existence are the main cause for the differences between parents and their offspring, can be demonstrated. He continues, that natural selection chooses the fittest, because otherwise, the earth could not hold the offspring of just a single pair. Therefore, DARWIN concludes, only few individuals can stay alive to propagate their species.
He begins his chain of proof with his own observations of cultural plants and domestic animals pointing out, how flexible a species behaves, and what possibilities man has to select and cultivate one or the other variety depending on his needs. He cites wheat as an example showing, how – despite the fact, that wheat belongs to the oldest cultivated plants of mankind - new varieties do still appear.
DARWIN’s following description shows, how small the changes occurring due to (human) selection are, and how many of these small changes have to be accumulated in order to obtain a perfect result:
"The pear, though cultivated in classical times, appears, from Pliny's description, to have been a fruit of very inferior quality. I have seen great surprise expressed in horticultural works at the wonderful skill of gardeners, in having produced such splendid results from such poor materials; but the art, I cannot doubt, has been simple, and, as far as the final result is concerned, has been followed almost unconsciously. It has consisted in always cultivating the best known variety, sowing its seeds, and, when a slightly better variety has chanced to appear, selecting it, and so onwards. But the gardeners of the classical period, who cultivated the best pear they could procure, never thought what splendid fruit we should eat; though we owe our excellent fruit, in some small degree, to their having naturally chosen and preserved the best varieties they could anywhere find."
He notices, that adaptation favouring not the benefit of a species, but the use and interest of man is a peculiarity of domestic races. Although he knew nothing about modern genetics, DARWIN concludes from different observations, that inheritance has to exist. He argues:
He notices, that adaptation favouring not the benefit of a species, but the use and interest of man is a peculiarity of domestic races. Although he knew nothing about modern genetics, DARWIN concludes from different observations, that inheritance has to exist. He argues:
"When a deviation appears not unfrequently, and we see it in the father and child, we cannot tell whether it may not be due to the same original cause acting on both; but when amongst individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in the parent -- say, once amongst several million individuals -- and it reappears in the child, the mere doctrine of chances almost compels us to attribute its reappearance to inheritance."
He discusses the nature of varieties and species without finding a clear definition, though he points out, that varieties can usually interbreed, while he is not able to give an example for a fully fertile hybrid of two different species. He uses the term ‘good’ or ‘true’ species, whenever the demarcation between species is clear. Species are often polymorphic, and he gives examples from the genera Rubus, Rosa and Hieracium.
He recognizes the connection between the number of species a genus has and the number of individuals of a species:
"From looking at species as only strongly-marked and well-defined varieties, I was led to anticipate that the species of the larger genera in each country would oftener present varieties, than the species of the smaller genera; [...] We have, also, seen that it is the most flourishing and dominant species of the larger genera which on an average vary most; and varieties, as we shall hereafter see, tend to become converted into new and distinct species. The larger genera thus tend to become larger; and throughout nature the forms of life which are now dominant tend to become still more dominant by leaving many modified and dominant descendants."
The struggle for existence or, as H. SPENCER put it, the ‘survival of the fittest’, does not only describe the survival of the individual, but even more the successful passing on of offspring.
Species subject to great variations in food, produce an especially large number of eggs and sperm. The individuals develop rather fast, so that reproduction occurs after only a short period. Despite this, a large part of the offspring dies, usually in the first phase of life.
Animals and plants able to protect their progeny produce rather few offspring (see also r-strategy and K-strategy).
The struggle for existence means, too, an adaptation to other species. A certain English species of Lobelia, for example, is in nature never visited by insects. Propagation by seed does therefore not occur. DARWIN was able to generate a lot of seed by artificial fertilisation. He uses this example to point out the importance of insects in the pollination of plants and explains mechanisms of adaptation between plants and insects. The length and curvature of an insect’s proboscis determines the plants it can pollinate. DARWIN concludes, that without the dependence of plants on pollinating insects, the great variety of flower colours and shapes would not exist. Wind-pollinated plants have no conspicuous flowers.
Another important influence in the struggle for existence is sexual selection:
" And this leads me to say a few words on what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. "
Species becoming rare
Doubts about the theory of selection remained despite the many supporting instances. Most referred to the evolution of animals and will thus not be reviewed here, but an important question is, why so little transitional stages between species exist, although each species is supposed to have developed from another species by very small steps. DARWIN replies that, if each species is regarded as a descendant of some unknown form, then the original variety and the transitional stages are usually already extinct due to the process of development and perfection of the new forms.
Why does the hybridisation of varieties usually produce fertile hybrids, while hybridisations of species make for non-fertile offspring ? DARWIN assumes, that in the latter case the organisation of the individual is disturbed due to the fusion of two species.
The capture and analysis of fossils produces instances for the process of evolution. DARWIN points out, that palaeontological collections are usually poor – and I would like to mention that this is still the case - and that nobody can expect to find all the postulated transitional stages. Geological data do nevertheless supply the facts for important conclusions like
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the successive appearance of new species, |
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the different velocities with which the features of different species change, |
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the extinction of species and the fact that extinct species do never recur due to the terminated row of generations, |
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the conclusion that all life forms on earth change almost simultaneously, and |
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the fact that extinct species were related with each other and are related with living species. |
The extinction of old forms is regarded as an almost unavoidable consequence of the development of new ones. The extinction of whole groups of species is often a slow process, because single species may drag on to exist in protected or isolated habitats. In each of the subsequent periods of the earth’s history, the inhabitants have defeated their predecessors in the struggle for existence and are thus on a higher level of perfection. Their anatomy is usually more specialized.
In the concluding chapters, DARWIN describes geographic characteristics as important factors of selection stressing especially the peculiarities of animals and plants on small oceanic islands. He proves with convergence, how important the adaptation and the development of certain features are: mice and the marsupial mice of Australia – they are not related – are of almost the same shape.
Mimicry is another example quoted by DARWIN. Certain species take on the shape and colouring of other species in order to be protected against enemies. In his final observation, he predicts:
"In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history. "
These two botanical essays of DARWIN present a whole host of supplementing proofs for the theory of selection. Above all, they focus on the advantages and mechanisms of sexual propagation. DARWIN regards the great variety of orchid flowers as a result of the adaptation to their pollinators, i.e. insects. No other plant family developed and perfected as many impressive differences in flower structure, while keeping a large degree of uniformity of the plants’ vegetative parts. The flowers are built to allow, favour or even demand cross-fertilisation. Despite their extreme specialisation, pollination is by no means always secure.
DARWIN cites observations H. MÜLLER made on South Brazilian species that are not visited by insects and do consequently also produce no seed. An Ophrys species growing in England is self-fertile despite conspicuous flowers. He interprets this phenomenon as a degeneration. His work on cross- and self-fertilisation is for the most part a protocol of the results of extensive series of experiments. At the beginning of "The various contrivances..", he acknowledges C. K. SPRENGEL’s study
"The revealed secret of nature of flower structure and pollination"
This study (1793) was ahead of its time and was therefore little noticed when published. It was the first study to point out the importance of insects in the fertilisation of plants.
DARWIN compared the success of self- and cross-fertilisation under controlled experimental conditions. Self-fertilisation, also called autogamy or selfing is the fertilisation of a plant with its own pollen. Cross-fertilisation or allogamy is the fertilisation with pollen of another plant of usually the same species.
The success of fertilisation can be measured as the number of produced ripe seed and as a number of different properties like size, weight or fertility of the progeny. The analysis of experimental data from over 50 plant species led DARWIN to the conclusion that allogamy is more favourable in by far the most cases.
Several species (Mimulus luteus, Ipomoea purpurea, Dianthus caryophyllus, Petunia violacea among others) showed almost uniform flower colour and flower structure as a result of self-fertilisation, while allogamy led to a variety of variations.
In many species, self-fertilisation causes a considerable reduction of fertility, in the case of Eschscholzia californica it leads to a reduction to only 15 percent, while fertility remains unchanged in others (Lobelia fulgens or Gesneria pendulina). A few species propagate almost exclusively by self-fertilisation, others are self-incompatible or obligatorily cross-fertilising. Among the latter are Verbascum phoeniceum and Verbascum nigrum. The closely related species Verbascum thapsus and Verbascum lychnitis use both types of fertilisation successfully. Such differences occur, too, in the genera Papaver and Corydalis.
Reseda odorata stands out due to an intraspecific, individual variability concerning fertilisation. Some specimen are able to self-fertilize, others not. In summary, DARWIN writes, that the most important conclusion he made, is, that the mere act of hybridisation is not favourable by itself. A positive result, he resumes, depends on the fact, that the specimen to be hybridized differ slightly in their constitution as a consequence of their ancestors being subject to slightly different conditions also known as ‘spontaneous changes’.
Finally, DARWIN looks into the hermaphrodism of plants, i.e. the fact that plants have female and male reproductive organs in one flower. He explains hermaphrodism as consequence of the danger of originally dioecious plants, that had formerly the advantage to be always fertilized by other specimen, to remain not fertilized and thus to produce no progeny. With dioecious plants, female flowers (with carpels) and male flowers (with stamina) belong never to the same plant.
The disadvantages of self-fertilisation are compensated as plants are – in contrast to most animals – immobile and can thus not look actively for a partner.
Dioecious flowers are more common in trees than in herbs, since – due to the longevity of trees - they offer a better compensation for a poor or even failing fruit development potentially happening in some years. The topic isolation and reproduction, and mechanisms of reproduction shows that the evolution and dispersal of annual herbs is so successful, because these plants use the different types of propagation, i.e. self-fertilisation, cross-fertilisation, and vegetative reproduction with equal efficiency.
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