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GLOSSARY 



OF THB 



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 he 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 ravs 

 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 cha- 

 racteristic of the species have not been produced in the skin and its 

 appendages. Albinism is the state of being an albino. 



ALOJE. 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. The 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. 



* 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 givo 

 the explanations of the terms in as popular a form as possible. 



