XXXll INTRODUCTION. 



I. Metamorpliic, advocated by Swammerdam and 

 Lamarck, which deals principally with larval 

 changes. 

 II. Cibarian, advocated by Fabricius, which deals 

 chiefly with the anatomy of the mouth parts. 

 III. Alary, employed by Linn^us and De Geer, who 

 laid stress on the importance of the wing- 

 veining and the development of its details. 

 But however convenient such systems may be, we 

 have no sure proof that they are natural ; for some- 

 times animals, and not excepting insects which are 

 nearly allied to each other in form, materially differ in 

 their habits and metamorphoses ; and such exceptional 

 examples do not accommodate themselves to schemes 

 which some have hence thought arbitrary and unreal. 

 In erecting a Natural Biological System numerous 

 considerations must be taken into account. Not only 

 should physical form, colour, modes of reproduction, 

 and life-economy be taken into thought, but to these 

 must be added the important but difficult questions 

 rising out of Phyllogeny and modern iEtiology, which 

 last study treats of the laws of variation. 



Our certain knowledge of the descent of group from 

 group shadowed forth in the phenomena of Embry- 

 ology, and the results of discovery bearing on the 

 laws of variation, are as yet too scanty to help us 

 much, as simple systematists. 



But it is more easy to point out defects in systems 

 than to construct others on more natural bases. 

 Examples of such difficulties are not hard to find. Thus 

 amongst insects we find Lepidoptera with mandibulate 

 larvae and proboscidean images ; yet the latter may 

 not be classed with the Ehynchota. Again, in the 

 Ephemeridae we find most commonly four-winged, and 

 more rarely two-winged, images ; yet the last are not 

 Diptera. Amongst the Vertebrata we might expect to 

 find in a genus a constant in the number of vertebrae ; 

 yet amongst pigeons we have strange anomalies in 

 this respect, and we find discrepancies even amongst 



