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GLOSSARY 



PBINCIPAL SCIENTIFIC TEEMS USED IN THE 

 PEESENT VOLUME.* 



A BERRANT. — Forms or groups of animals or plants which deviate in im- 

 portant characters from their nearest allies, so as not to be easily in- 

 cluded 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. 



Alg^e. — A class of plants including the ordinary sea-weeds and the fila- 

 mentous 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 cham- 

 bers waved in complicated patterns at their junction with the outer 

 wall of the shell. 



Analogy. — That resemblance of structures which depends upon simi- 

 larity 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. TV. S. Dallas for this Glossary, which has been 

 given because several readers have complained to me that some of the terms used weie 

 unintelligible to them. Mr. Dallas has endeavoured to give the explanations of the 

 terms in as popular a form as possible. 



