rganisms, or even all objects organic or inorganic, can be arranged
in a single ascending series. The idea is a common one; its first
literary expression is found perhaps in primitive creation-myths, in
which inorganic things are created before organic, and plants before
animals. It may be recognised also in Anaximander's theory that land
animals arose from aquatic animals, more clearly still in Anaxagoras'
theory that life took its origin on this globe from vegetable germs
which fell to earth with the rain. Anaxagoras considered animals
higher in the scale than plants, for while the latter participated in
pleasure (when they grew) and pain (when they lost their leaves),
animals had in addition "Nous." In Empedocles' theory of evolution,
the vegetable world preceded the animal. Plato, in the _Timaeus_,
describes the whole organic world as being formed by degradation from
man, who is created first. Man sinks first into woman, then into brute
form, traversing all the stages from the higher to the lower animals,
and becoming finally a plant. This is a reversal of the more usual
notion, but the idea of gradation is equally present.

Aristotle seems not to have believed in any transformation of species,
but he saw that Nature passes gradually from inanimate to animate
things without a clear dividing line. "The race of plants succeeds
immediately that of inanimate objects" (Cresswell, _loc. cit._, p.
94). Within the organic realm the passage from plants to animals is
gradual. Some creatures, for example, the sea-anemones and sponges,
might belong to either class.

Aristotle recognised also a natural series among the groups of
animals, a series of increasing complexity of structure. He begins his
study of structure with man, who is the most intricate, and then takes
up in turn viviparous and oviparous quadrupeds, then birds, then
fishes. After the Sanguinea he considers the Exsanguinea, and of the
latter first the most highly organised, the Cephalopods, and last the
simplest, the lower members of his class of the Testacea. In treating
of generation (in _Hist. Animalium_, v.) he reverses this order. In
the _De Generatione_ (Book ii., I) there is given another serial
arrangement of animals, this time in relation to their manner of
reproduction. There is a gradation, he says, of the following kind:--

1. Internally viviparous Sanguinea } producing a perfect
2. Externally viviparous Sanguinea } animal.
3. Oviparous Sang