Sunday, 6 January 2008

Linnaeus - Princeps Botanicorum

There are very few examples of scientific terminology that have become sufficiently well-known to have become a part of popular culture. The chemical formula for water – H2O – is certainly one; it is so familiar it has even featured in advertisements. Another is the equation E = mc squared – while not everybody knows that it defines a relationship between mass and energy, most will have heard of it and will be aware it was formulated by Albert Einstein.

But the most familiar scientific term of all has to be Homo sapiens – Mankind’s scientific name for himself.

The term was originated by the 18th Century Swedish scientist Carl von Linné (1707-78), better known as Linnaeus, who first formally “described” the human species in 1758. It means (some would say ironically!) “wise man” or “man the thinker”. It is an example of what biologists call the binomial nomenclature, a system whereby all living things are assigned a double-barrelled name based on their genus and species. These latter terms are in turn part of a bigger scheme of classification known as the Linnaean taxonomy, which – as the name implies – was introduced by Linnaeus himself.

Man has been studying and classifying that natural world throughout recorded history and probably much longer. A key concept in classification of living organisms is that they all belong to various species, and this is a very old idea indeed, almost certainly prehistoric in origin. For example, it would have been obvious that sheep all look very much alike, but that they don’t look in the least bit like pigs, and that therefore all sheep belong to one species and all pigs belong to another. Today we refer to organisms so grouped as morphological species.

In addition, the early Neolithic farmers must soon have realised that while a ewe and a ram can reproduce, and likewise a sow and a boar; a ewe and a boar, or a sow and a ram cannot. Sheep and pigs are different biological species, though this definition of a species was not formalised until much later, by John Ray (1628-1705), an English naturalist who proclaimed that “one species could never spring from the seed of another”.

The first attempt at arranging the various species of living organisms into a systematic classification was made by the Greek philosopher Aristotle (384-322 BC), who divided them into two groups – animals and plants. Animals were further divided into three categories - those living on land, those living in the water and those living in the air, and were in addition categorised by whether or not they had blood (broadly speaking, those “without blood” would now be classed as invertebrates, or animals without a backbone). Plants were categorised by differences in their stems.

Aristotle’s system remained in use for hundreds of years but by the 16th Century, Man’s knowledge of the natural world had reached a point where it was becoming inadequate. Many attempts were made to devise a better system, with some notable works being published by Conrad Gessner (1516-65), Andrea Cesalpino (1524-1603) and John Ray (1628-1705).

In addition Gaspard Bauhin (1560-1624) introduced the binomial nomenclature that Linnaeus would later adopt. Under this system, a species is assigned a generic name and a specific name. The generic name refers to the genus, a group of species more closely related to one another than any other group of species. The specific name represents the species itself. For example lions and tigers are different species, but they are similar enough to both be assigned to the genus Panthera. The lion is Panthera leo and the tiger Panthera tigris.

Despite these advances, the science of biological classification at the beginning of the 18th Century remained in a confused state. There was little or no consensus in the scientific community on how things should be done and with new species being discovered all the time, the problem was getting steadily worse.

Step forward Carl Linné, who was born at Rashult, Sweden, in 1707, the son of a Lutherian curate. He is usually known by the Latinised version of his name, Carolus Linnaeus. It was expected that young Carl would follow his father into the Church, but he showed little enthusiasm for this proposed choice of career and it is said his despairing father apprenticed him to a local shoemaker before he was eventually sent to study medicine at the University of Lund in 1727. A year later, he transferred to Uppsala. However his real interest lay in Botany (the study of plants) and during the course of his studies he became convinced that flowering plants could be classified on the basis of their sexual organs – the male stamens (pollinating) and female pistils (pollen receptor).

In 1732 he led an expedition to Lapland, where he discovered around a hundred new plant species, before completing his medical studies in the Netherlands and Belgium. It was during this time that he published the first edition of Systema Naturae, the work for he is largely remembered, in which he adopted Gaspard Bauhin’s binomial nomenclature, which to date had not gained popularity. Unwieldy names such as physalis amno ramosissime ramis angulosis glabris foliis dentoserratis were still the norm, but under Bauhin’s system this became the rather less wordy Physalis angulata.

This work also put forward Linnaeus’ taxonomic scheme for the natural world. The word taxonomy means “hierarchical classification” and it can be used as either a noun or an adjective. A taxonomy (noun) is a tree structure of classifications for any given set of objects with a single classification at the top, known as the root node, which applies to all objects. A taxon (plural taxa) is any item within such a scheme and all objects within a particular taxon will be united by one or more defining features.

For example, a taxonomic scheme for cars has “car” as the root node (all objects in the scheme are cars), followed by manufacturer, model, type, engine size and colour. Each of these sub-categories is known as a division. An example of a car classified in the scheme is Car>Ford>Mondeo>Estate>2.3 Litre>Metallic silver. An example of a taxon is “Ford”; all cars within it sharing the defining feature of having been manufactured by the Ford Motor Company.

The taxonomy devised by Linnaeus, which he refined and expanded over ten editions of Systema Naturae, had six divisions. At the top, as in the car example is the root note, which Linnaeus designated Imperium (Empire), of which all the natural world is a part. The divisions below this were Regnum (Kingdom), Classis (Class), Ordo (Order), Genus and Species.

The use of Latin in this and other learned texts is worth a brief digression. At the time few scientists spoke any contemporary language beyond their own native tongue, but most had studied the classics and so nearly all scientific works were published in Latin, including Sir Isaac Newton’s landmark Philosophiae Naturalis Principia Mathematica (The Philosophy of Natural Mathematical Principles) and Linnaeus’ own Systema Naturae. One notable exception was Galileo’s The Dialogue Concerning the two Chief World Systems, aimed at a wider audience and thus an early example of “popular science” (though it certainly wasn’t very popular with the Inquisition!).

Linnaeus recognised three kingdoms in his system, the Animal kingdom, the Plant Kingdom and the Mineral Kingdom. Each kingdom was subdivided by Class, of which the animal kingdom had six: Mammalia (mammals), Aves (birds), Amphibia (amphibians), Pisces (fish), Insecta (insects) and Vermes (worms). The Mammalia (mammals) are those animals that suckle their young. It is said that Linnaeus adopted this aspect as the defining feature of the group because of his strongly-held view that all mothers should breast feed their on babies. He was strongly opposed to the then-common practice of “wet nursing” and in this respect he was very much in tune with current thinking.

Each class was further subdivided by Order, with the mammals comprising eight such orders, including the Primates. Orders were subdivided into Genera, with each Genus containing one or more Species. The Primates comprised the Simia (monkeys, apes, etc) and Homo (man), the latter containing a single species, sapiens (though Linnaeus initially also included chimpanzees and gibbons).

The Linnaean system did not accord equal status to apparently equal divisions; thus the Mineral Kingdom was ranked below the Plant Kingdom; which in turn sat below the Animal Kingdom. Similarly the classes were assigned ranks with the mammals ranking the highest and the worms the lowest. Within the mammals the Primates received top billing, with Homo sapiens assigned to pole position therein.

This hierarchy within a hierarchy reflected Linnaeus’ belief that the system reflected a Divine Order of Creation, with Mankind standing at the top of the pile and indeed the term “primate” survives to this day as a legacy of that view. It should be remembered that the prevalent belief at the time of Linnaeus was that the Earth and all living things had been produced by God in their present forms in a single act. This view, now known as Creationism, wasn’t seriously challenged until the 19th Century.

Linnaeus’ system was an example of natural theology, which is the study of nature with a view to achieving a better understanding of the works of God. It was heavily relied on by the deists of that time. Deists believe that knowledge of God can be deduced from nature rather than having to be revealed directly by supernatural means. Deism was very popular in the 18th Century and its adherents included Voltaire, Thomas Jefferson and Benjamin Franklin.

Though some were already beginning to question Creationism, Linnaeus was not among them and he proclaimed that “God creates, Linnaeus arranges”. It has to be said that modesty wasn’t Linnaeus’ strongest point and he proposed that Princeps Botanicorum (Prince of Botany) be engraved on his tombstone. He was no doubt delighted with his elevation to the nobility in 1761, when he took the name Carl von Linné.

Linnaeus did have his critics and some objected to the bizarre sexual imagery he used when categorising plants. For example, “The flowers' leaves...serve as bridal beds which the Creator has so gloriously arranged, adorned with such noble bed curtains, and perfumed with so many soft scents that the bridegroom with his bride might there celebrate their nuptials with so much the greater solemnity...”. The botanist Johann Siegesbeck denounced this "loathsome harlotry" but Linnaeus had his revenge and named a small and completely useless weed Siegesbeckia! In the event Linnaeus’ preoccupation with the sexual characteristics of plants gave poor results and was soon abandoned.

Nevertheless, Linnaeus’ classification system, as set out in the 10th edition of Systema Naturae, published in 1758, is still is considered the foundation of modern taxonomy and it has been modified only slightly.

Linnaeus continued his work until the early 1770s, when his health began to decline. He was afflicted by strokes, memory loss and general ill-health until his death in 1778. In his publications, Linnaeus provided a concise, usable survey of all the world's then-known plants and animals, comprising about 7,700 species of plants and 4,400 species of animals. These works helped to establish and standardize the consistent binomial nomenclature for species, including our own.

We have long ago discarded the “loathsome harlotry” and the rank of Empire. Two new ranks have been added; Phylum lies between Kingdom and Class; and Family lies between Order and Genus, giving seven hierarchical ranks in all. In addition, prefixes such as sub-, super-, etc. are sometimes used to expand the system. (The optional divisions of Cohort (between Order and Class) and Tribe (between Genus and Family) are also sometimes encountered, but will not be used here). The Mineral Kingdom was soon abandoned but other kingdoms were added later, such as Fungi, Monera (bacteria) and Protista (single-celled organisms including the well-known (but actually quite rare) Amoeba) and most systems today employ at least six kingdoms.

On this revised picture, Mankind is classified as follows:

Kingdom: Animalia (animals)
Phylum: Chordata (possessing a stiffening rod or notochord)
Sub-phylum: Vertebrata (more specifically possessing a backbone)
Class: Mammalia (suckling their young)
Order: Primates (tarsiers, lemurs, monkeys, apes and humans)
Family: Hominidae (the Hominids, i.e. modern and extinct humans, the extinct australopithecines and, in some recent schemes, the great apes)
Genus: Homo
Species: sapiens

It should be noted that we while we now regard all equivalent-level taxa as being equal, the updated scheme would work perfectly well if we had continued with Linnaeus’ view that some taxa were rather more equal in the eyes of God than others, and it is in no way at odds with the tenets of Creationism. The Linnean Taxonomy shows us where Man fits into the grand scheme of things, but it has nothing to tell us about how we got there. It was left for Charles Darwin to point the way.

© Christopher Seddon 2008

No comments: