ON INSECT ANATOMY. 391 



and unscientific fancies have undergone frequent remoulding upon 

 the more precise indications of anatomical structure, and have 

 improved in proportion as our knowledge of the relation between 

 insect-life habits and their external and internal organisation becomes 

 more accurate. It was not, for instance, until Cuvier demonstrated 

 the broad distinctions in the organs of vegetable life, that the 

 incongruous animals once associated under the heading ''Insecta" 

 were arranged in their respective divisions of Crustacea, Aiachnida, 

 Myriapoda, and Insecta, now defined by striking contrasts of 

 circulating and respiratory apparatus by different number and 

 arrangement of body segments, limbs, &c. &c., and by distinctive 

 characters of sensory and nerve organs. 



Bat in the classification of insects, properly so-called, physio- 

 logical anatomy has been less happily applied. The division of 

 insects into groups, according to external signs of metamorphosis 

 does not correspond with any principle of natural affinities between 

 the members of the order so constituted. 



Metamorphosis as a basis of classification necessarily fails when 

 the visible changes differ so greatly in degree amongst animals 

 nearest in affinity. And, moreover, its real significance as an 

 indication of the process of evolution through which all creatures 

 pass was so imperfectly recognised by those who first employed 

 insect metamorphosis as a means of classification, that two of the 

 three divisions were founded on a more or less absolute negation of 

 metamorphic phenomena. So far as the unequal prominence of 

 visible metamorphic changes may assist the entomologist in dis- 

 tinguishing his orders, it might be accepted for what it is worth; 

 But the true anatomical expression of metamorphosis points rather 

 to resemblances than differences to homologies than analogies to 

 homogeneity of structure rather than heterogeneity. For as in its 

 physiological aspect, metamorphosis reveals an underlying unity of 

 action throughout the whole animal kingdom, so from its anatomical 

 analysis we discover unity of structural elements, and their modes 

 of evolution — the differentiations which ultimately ensue being the 

 result of special individual conditions. In short, metamorphosis 



