The Origin of Species

Glossary

I am indebted to the kindness of Mr. W. S. Dallas for this Glossary, which has been given because several readers have complained to me that some of the terms used were unintelligible to them. Mr. Dallas has endeavoured to give the explanations of the terms in as popular a form as possible.

EDITOR'S NOTE: This glossary did not appear in the first edition and has been reproduced here directly from the sixth edition

 ABERRANT

Forms or groups of animals or plants which deviate in important characters from their nearest allies, so as not to be easily included in the same group with them, are said to be aberrant.

 ABERRATION (in Optics)

In the refraction of light by a convex lens the rays passing through different parts of the lens are brought to a focus at slightly different distances, this is called spherical aberration; 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 earthworms, and the leeches.

 ANTENNÆ

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

Having the two sides unlike.

 ATROPHIED

Arrested in development at a very early stage.

 BALANUS

The genus including the common Acorn-shells which live in abundance on the rocks of the sea-coast.

 BATRACHIANS

A class of animals allied to the Reptiles, but undergoing a peculiar metamorphosis, in which the young animal is generally aquatic and breathes by gills. (Examples, Frogs, Toads, and Newts.)

 BOULDERS

Large transported blocks of stone generally imbedded in clays or gravels.

 BRACHIOPODA

A class of marine Mollusca, or soft-bodied animals, furnished with a bivalve shell, attached to submarine objects by a stalk which passes through an aperture in one of the valves, and furnished with fringed arms, by the action of which food is carried to the mouth.

 BRANCHIÆ

Gills or organs for respiration in water.

 BRANCHIAL

Pertaining to gills or branchiæ.

 CAMBRIAN SYSTEM

A Series of very ancient Palæozoic rocks, between the Laurentian and the Silurian. Until recently these were regarded as the oldest fossiliferous rocks.