Seite - 531 - in The Origin of Species

GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME* 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 char- acteristic of the species have not been produced in the skin and its appendages. Albinism is the state of bemg an albino. AlgcB—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 fowler 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 exist- ing pearly Nautilus, but having the partitions between the chambers waved in complicated patterns at their junction with the outer wall of the shell. Analogy—The resernblance 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. AnitnaJcule—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 pro- vided with appendages for locomotion and with gills. It includes the ordinary marine worms, the earthworms, and the leeches. Antenna— .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 fertilizing 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 organized. Articulata—A great division of the Animal Kingdom characterized 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. * 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 endeav- oured to give the explanations of the terms in as popular a form as possible. 531