VI PEEFACE. 



of definitions might possibly have led to an abuse of 

 what is really intended to be a mere guide to a line of 

 study, and not a thing to be studied in itself. 



While definitions have been omitted, a limited num- 

 ber of illustrations have been introduced, as self-ex- 

 planatory of the text. Occasionally, also, remarks on 

 doubtful points, or divergent views as to classification, 

 are introduced, or alternative arrangements are sub- 

 mitted ; and there are added to each group references 

 to some of the sources of special information, which 

 can be studied by advanced students. It seems hardly 

 necessary to add that the purpose of such a classifica- 

 tion as is here given, is not that it should be, even in 

 parts, committed to memory, but simply that it may 

 serve as a skeleton, which the student must endow 

 with life by his own work. 



MARISCHAL COLLEGE, ABERDEEN, 

 July 5, 1882. 



