Plants and animals are classified according to
their resemblances and they are placed in one or other of a not
very large number of groups such as ferns, conifers, molluscs, or
mammals. But within each of these groups, there is subdivision into
other smaller groups, mammals being so subdivided into rodents,
carnivores, ungulates, and primates for example. Within these again
there is further subdivision, and the important point to notice is
that Classification always places species in groups that are
contained within other larger groups. This is so common that its
significance is often overlooked. Why do organisms have to be
classified like this ? Why are they not strewn in single file up
the ladder of the plant and animal kingdoms, or fortuitously like
pebbles on a beach, or arbitrarily like the stars imaginary
constellations ? The reason is that the arrangement of groups
within groups is a natural classification reflecting the course of
evolution. It is the result of descent from common ancestors and
indication of affinity; the differences between the groups are due
modification and divergence during such descent.
In a group classified in accordance with a
natural system of classification it is possible to show the
affinities and derivations of the various sub-groups in the form of
a diagram or evolutionary tree. The resemblance in structure
between different living organisms indicates the degree to which
they are related, which is the basis of biological classification.
Man is a warm-blooded, air-breathing animal with hairy skin, whose
young are born alive and fed at the breast. He is therefore a
mammal.
When Man's bodily structure is compared in detail
with that of other mammals, it is clear that he belongs to the
order of Primates, distinguished by large brains, and grasping
hands and fingers with nails rather than claws. The Primates also
include tree-shrews, lemurs, tarsiers, monkeys, and apes. Thomas
Henry Huxley in one of his essays on Mans Place in Nature (1894),
wrote: 'Whatever system of organs be studied ... the structural
differences which separate Man from the Gorilla and Chimpanzee are
not so great as those which separate the Gorilla from the lower
apes' (i.e. monkeys).
Much of the work of eighteenth-century biologists
concerned arranging species into taxonomies. Part of the urgency of
this project arose from the sheer number of species discovered: in
antiquity, Theophrastus could identify five hundred plant species;
by the late Renaissance, Bauhin could identify over six thousand;
Linnaeus catalogued eighteen thousand; and Cuvier listed over
fifty thousand separate species of plants. Although most earlier
botanists had been content merely to describe individual species,
natural philosophers of the late seventeenth and early eighteenth
centuries began to see a need to arrange them into meaningful
categories. Newton's classification of the heavenly bodies in
Principia Mathematica (1687) increased the taxonomic urge in
biologists at the beginning of the eighteenth century.
The classification of species, however, began
well before the eighteenth century. Aristotle distinguished species
by habitat and means of reproduction, but Andrea Cesalpino produced
the first significant taxonomy of plants in 1583, arranging the
species in a hierarchical, graded order. His work was developed by
Marcello Malpighi, who expanded his hierarchical system to include
animals.
The English naturalist John Ray was the first to
formulate the idea of species. His late seventeenth-century
work is based on the taxonomy of Aristotle, but he provided a
sounder scientific basis on which to make distinctions of various
plants and animals from one another.
The single most important development in
taxonomy, however, came from the Swedish botanist Linnaeus, who in
1737 superseded Ray by publishing the taxonomic system which is the
basis of that used today. Whereas most previous taxonomies worked
by beginning with large categories subdivided along logical lines,
Linnaeus worked in the opposite direction, beginning empirically
with individual species and grouping them according to their
similarities. He borrowed Bauhin's system of nomenclature, and
identified twenty-four classes of plants, categorizing them by the
number of pistils and stamens: this classification based on
reproductive organs soon became the standard system. Later he
identified six classes of animals (quadrupeds, birds, amphibians,
fishes, insects, and worms), following the work of Ray.
Although Linnaeus established the basis of
taxonomy still in use today, those who followed him refined and
corrected his sytem. Among the most important was Jean- Baptise
Lamarck, who was unsatisfied with Linnaeus's classification of
invertebrates into only two classes, insects and worms. Lamarck
distinguished mollusks, arthropods, crustaceans, insects, and other
classes.
The classification of species carried with it
considerable political and theological baggage. Voltaire, for
instance, likened the hierarchical arrangement of species to
political and religious hierarchies in the Philosophical
Dictionary (1764). The debate became only more heated as
taxonomy became entangled in arguments over evolution, as
biologists began to explain both the similarities and differences
of the species by placing them at different points along an
evolutionary path. Early attempts to reconcile taxonomy with
evolution were tentative and inconsistent: lacking an adequate
empirical record (such as fossils), many theorists made wild
speculations about the relationships between the species. But as
the nineteenth century progressed and more evidence was collected,
the gaps were filled and scientists reconciled the theory and the
evidence without violence to either.