RATION; at
the same time the coloured rays are separated by the prismatic action of
the lens and likewise brought to a focus at different distances--this is
CHROMATIC ABERRATION.

ABNORMAL.--Contrary to the general rule.

ABORTED.--An organ is said to be aborted, when its development has been
arrested at a very early stage.

ALBINISM.--Albinos are animals in which the usual colouring matters
characteristic of the species have not been produced in the skin and its
appendages. Albinism is the state of being an albino.

ALGAE.--A class of plants including the ordinary sea-weeds and the
filamentous fresh-water weeds.

ALTERNATION OF GENERATIONS.--This term is applied to a peculiar mode of
reproduction which prevails among many of the lower animals, in which
the egg produces a living form quite different from its parent, but from
which the parent-form is reproduced by a process of budding, or by the
division of the substance of the first product of the egg.

AMMONITES.--A group of fossil, spiral, chambered shells, allied to the
existing pearly Nautilus, but having the partitions between the chambers
waved in complicated patterns at their junction with the outer wall of
the shell.

ANALOGY.--That resemblance of structures which depends upon similarity
of function, as in the wings of insects and birds. Such structures are
said to be ANALOGOUS, and to be ANALOGUES of each other.

ANIMALCULE.--A minute animal: generally applied to those visible only by
the microscope.

ANNELIDS.--A class of worms in which the surface of the body exhibits
a more or less distinct division into rings or segments, generally
provided with appendages for locomotion and with gills. It includes the
ordinary marine worms, the earth-worms, and the leeches.

ANTENNAE.--Jointed organs appended to the head in Insects, Crustacea and
Centipedes, and not belonging to the mouth.

ANTHERS.--The summits of the stamens of flowers, in which the pollen or
fertilising dust is produced.

APLACENTALIA, APLACENTATA or APLACENTAL MAMMALS.--See MAMMALIA.

ARCHETYPAL.--Of or belonging to the Archetype, or ideal primitive form
upon which all the beings of a group seem to be organised.

ARTICULATA.--A great division of the Animal Kingdom characterised
generally by having the surface of the body divided into rings called
segments, a greater or less number of which are furnished with jointed
legs (such as Insects, Crustaceans and Centipedes).

ASYMMETRICAL.--