CHAPTER XII 



EVOLUTION 



The scheme of Classification briefly set forth in the pre- 

 ceding chapter is an attempt to demonstrate the varying 

 degrees of likeness and divergence among insects, and it 

 has been repeatedly suggested in this book that these varying 

 degrees of likeness and divergence indicate varying degrees 

 of relationship. Long ago the systematic students of plants 

 and animals sought their ideal classification in a scheme 

 that might be regarded as '* natural." For the past sixty 

 years it has been generally realised among biologists that 

 *' relationship " between living creatures must not be con- 

 sidered merely a figure of speech, but that the degrees of 

 likeness and difference expressed in a schem.e of classification 

 indicate actual natural relationship. The general con- 

 ception of organic evolution became an accepted principle 

 among naturalists, as is well known, through the work of 

 Charles Darwin (1859), and a sentence from his famous 

 book may be quoted that sums up the meaning of systematic 

 study and its interpretation : "I believe that community 

 of descent is the bond which is partially revealed to us by 

 our classifications." In view of the immense number of 

 different species, genera, and families of insects, differing 

 in the most varied degrees among themselves, the student 

 of insect life finds a reasonable interpretation of the facts 

 of structure and classification in the principle of evolution. 

 The differences of habit and manner of life among insects 

 of the same group, as exemplified in their family and social 

 relations (see Chaps. VIII, IX), suggest that the creatures 

 may be plastic in their behaviour as well as modified in 



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