GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.

(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.)

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.

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