( 4) 
a progressive raising of the standard in both pursuer and 
pursued. This is why it is that insects mean so much to the 
naturalist or to the philosopher who desires to look beneath 
the surface for the forces which have moulded existing forms 
of life out of earlier and very different forms. The wings of 
butterflies, it has been said, serve as a tablet on which Nature 
writes the story of the modification of species.* But the 
careful study of insects tells us even more than this; for it 
gives us the clearest insight we as yet possess into the forces 
by which these changes have been brought about. Light is 
thrown upon the causes to which organic evolution is due no 
less than upon the course which organic evolution has pursued. + 
And I think we shall find that a consideration of the 
numerous distinct categories of forms presented by the insect 
world is especially advantageous in an attack upon the difficult 
question—‘‘ What is a species?”, while properly-directed 
observation of insects, and experiments upon insects afford 
the most hopeful prospect of a final answer. 
And here I am compelled to say a word in defence of the 
Lepidoptera from this point of view. Undoubtedly it is most 
unfortunate that the obvious attractions of the group have 
led entomologists to neglect other Orders ; for this can be the 
only explanation why naturalists have so often preferred to 
do over again what others have done already, apparently 
oblivious of fields comparatively empty and unexplored. It 
must further be admitted, that the greater visibility of 
structure, and the more urgent necessity for the study 
of structure in other groups, render them better instruments 
of zoological education. But although the Lepidoptera are 
inferior in this respect, although they lack the unique interest 
of the Hymenoptera and the social Neuroptera, and can- 
not claim any of the respect due to venerable age like the 
Aptera, Orthoptera and Neuroptera—in spite of their many 
demerits they stand at the head, not only of all insects, but 
* H. W. Bates, quoted by A. R. Wallace in ‘‘ Natural Selection,” 
London, 1875, p. 132. The original passage may be found in ‘‘ The 
Naturalist on the Amazons” (London, pp. 347, 348 of the 1879 
edition). 
+ This justification for the study of insects was urged by the present 
writer in the Hope Reports, vol. iii, 1903, preface, pp. 4, 5. 
