The Star Garden
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French naturalist Georges-Louis Leclerc Comte de Buffon criticised Linnaeus' taxonomy in The Histoire Naturelle, first published in 1749. Buffon criticised Linnaeus for organising species into classes containing plants as different as "the elm and the carrot" and "the rose and the strawberry" (Buffon, pp.105). Linnaeus did not see this as a problem because he did not think that arbitrary and natural systems were mutually exclusive. Instead, Linnaeus argued that a natural system would only be revealed once his artificial system was complete. Buffon did not consider that taxonomy this artificial could be instrumental in finding a natural system and so believed that by ignoring nature, Linnaeus was moving further and further from the truth.
Buffon suggested that we should judge creatures based on the appearance of their whole bodies not just singular parts. He stated that individuals can be said to be of the same species only if they "resemble each other exactly, or if the differences between them are so small that they can only be perceived with difficulty" (Buffon, pp.106). In 1749, Buffon extended his taxonomy after considering how "we have altered, modified and changed the species of domestic animals" thereby creating "physical and real genera" (Buffon, pp.407).
Buffon wrote for a very different audience to Linnaeus, he was dependent upon the King to fund his research and upon the interest of aristocratic society, which was important in extending the patronage system. If The Histoire Naturelle had not appealed to aristocratic society then Buffon would not have been able to convert the Jardin du Roi, which began as a small medical garden, into the world's foremost centre for natural history or extend the number of professional chairs for naturalists in France (Spary, pp.16). The Histoire Naturelle was judged to be a masterpiece (Spary, pp.17) and Buffon's taxonomy remained popular until the French Revolution in 1789 (Spary, pp.18). After this Linnaeus' taxonomy became dominant again.
In 1859, English biologist Charles Darwin's theory of evolution by natural selection provided a new way to define species which was not constant in time (Darwin, pp.286). There are a number of definitions of species currently in use including the morphological approach, the biological approach and the phylogenetic approach.
The morphological approach defines species by the similarity of their anatomy, this is the concept most easily used by paleontologists, but not all species can be defined this way. Members of what we would consider the same species can vary enormously in appearance, especially if we consider the difference between males and females. It is also the case that many distinct species can be very similar physically (van Valen, pp.233-239).
The biological approach defines a species by its member's ability to interbreed, produce fertile offspring and be reproductively isolated from other species. One problem with this approach is that members of a species may be reproductively isolated due to geographical location but not differ physically. The main problem with universalising this approach, however, is that it cannot be applied to plants or other exclusively asexual species. This used to be seen as a small, and therefore acceptable, exception but we now know that the first half of life on Earth evolved without the need for sexual reproduction (Mayr, 1999).
This problem appears to be solved by the phylogenetic approach. This defines species by the genealogical relationship they share, their unique ancestry. It would apply to some asexual species because they exchange of genetic material (Mishler and Donoghue, pp.496), however it is difficult to apply to sexual organisms as it would require reclassifying accepted species. Birds, for example, would be categorised within the reptile group as they diverged from this branch before many other species that are currently accepted to be reptiles (Dupre, pp.17).
Species pluralists argue that there is no single definition of species that is applicable to all fields of biology and so there are a number of equally legitimate definitions. Pluralists can differ in motivation, epistemological pluralists argue that the term species is simply too complex for us to define. There may be one correct definition but we cannot uncover it and so must rely on a plurality of inaccurate descriptions. Ontological pluralists, on the other hand, argue that there is no single way to categorically define all life forms as species. This could be because the phylogenetic tree naturally diverges in numerous ways for numerous reasons. There is also no natural way to define when a parent becomes a different species to its offspring. It has been suggested that Darwin favoured ontological pluralism (Beatty, 1988).
It could be argued that the definition of species in biology is analogous to the definition of planet in astronomy. There are objects in the Kuiper belt which may or may not constitute planets. It would be impractical to include all of these, but by disallowing them, Pluto is also excluded. There is a separation of our intuitions whereby we want to include some things as planets but not others, based on instrumental reasons such as ease of communication.
A monistic definition has been chosen in the case of planets, not because planets are natural kinds, but because this is the easiest way to categorise in astronomy. As with species, the definition of planets is not restricted by our lack of understanding but by our knowledge that there are no objective boundaries between some objects.
One objection to pluralism is that it leads to confusion. Some argue that a monistic reference system is better than numerous inconsistent theories, even if it is man-made. The metric system and the periodic table are examples of this (Hull, pp.14). Yet other concepts, such as genes, have survived a plurality of definitions, as have many definitions used in physics (Mishler and Donoghue, pp.500).
The second objection to pluralism is that it is too liberal and allows for bad science. The absence of an objective species category would allow creationists and other non-scientists to define species in ways which will have the same merit as scientific definitions. The response to this is of course to define what make a scientific, rather than non-scientific, species definition. These criteria will be contingent upon historical contexts and so, as with the case of planets, different definitions will be correct in different times.
References
Beatty, J., 1988, 'Speaking of Species: Darwin's Strategy', The Darwinian Heritage, Kohn, D. (ed.), Princeton University Press, Princeton
Buffon, G.L.L., 1981, 'The Histoire Naturelle', Natural History to the History of Nature: Readings from Buffon and His Critics, Lyon, J. and P. Sloan, P., (trans.), University of Notre Dame Press, Paris
Darwin, C. R., 2001, 'On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life', Adamant Media Corporation, Boston
Dupre, J., 1999, 'On the Impossibility of a monistic account of species', Species: new interdisciplinary essays, Wilson, R.A. (ed.), MIT Press, Cambridge
Gottlieb, P., 2007, 'Aristotle on Non-contradiction', Stanford Encyclopedia of Philosophy
Hull, D, 1999, 'On the Plurality of species: questioning the party line', Species: new interdisciplinary essays, Wilson, R.A. (ed.), MIT Press, Cambridge
Koerner, L., 1996, 'Carl Linnaeus in his time and place', Cultures of Natural History, Jardine, N., Secord, J. and Spary, E.C. (eds.), Cambridge University Press, Cambridge, pp.145-162
Linnaeus, C., 1956, 'Systema naturae', British Museum, London
Mayr, E., 1999, 'Systematics and the origin of species, from the viewpoint of a zoologist', Harvard University Press, Harvard
Mishler, B. and Donoghue, M., 1982, 'Species Concepts: A case for Pluralism', Systematic Zoology, Vol.31, pp.491-503
Spary, pp.16E.C., 2000, 'The Place of the Histoire naturelle at the Jardin du Roi', Utopia's garden: French natural history from Old Regime to Revolution, University of Chicago Press, Chicago, pp.15-48
van Valen, L., 1976, 'Ecological species, multispecies, and oaks', Taxon, Vol.25
What are Species? (1700-1800s)
The meaning of the word 'species' has varied over time and there is currently no single accepted definition. Aristotle was a species monist, species monists argue that there is one true definition of the term species, even if we have not found it yet. Aristotle based his opinion on the theory of essentialism (Gottlieb, 2007). Essentialism is the view that species are natural kinds with essences. This means they are a spatiotemporally unrestricted group and all members contain an essence unique to their kind. This would be the optimal way to categorise objects since there would be no anomalies but it is not practical.
In 1735, Swedish naturalist Carl Linnaeus developed a taxonomy which could be easily used by amateurs (Linnaeus, 1956). Linnaeus was mostly self-taught and never displayed an interest in the exact sciences or modern technologies. His home was described as an emporium where "art and organic nature, parrots and squirrels, and even a young orangutan, played among potted plants, insect specimens, mineral samples, scientific instruments, and herbarium sheets" (Koerner, pp.153). After receiving a degree in medicine, Linnaeus began working in the Uppsala botanic garden and, in 1731, he began to publish catalogues based on his own taxonomy. Linnaeus' books were small enough to carry around and explained how to set up a herbarium, plant a garden and even embark on a voyage of discovery (Koerner, pp.147-148). His taxonomy was based on easily observable features and, in 1751, he began to name species after terms inspired by folklore and the bible (Koerner, pp.150).