I] INTRODUCTORY 3 



strangely fused together and modified. All that 

 has been so far said applies equally to fleas and to 

 other insects. 



It is of great interest, when one comes to make 

 a minute study of the form and external structure 

 of a flea, to try and trace the modifications that must 

 have taken place in the course of descent from the 

 ancestral arthropod ; but the relationship of fleas to 

 other insects living at the present day is of more 

 immediate concern. Insects are highly specialized 

 arthropods and fleas are highly specialized insects. 

 This means that they have become vastly modified 

 from the primitive ancestral type and fitted there- 

 by for a life among certain defined and peculiar 

 surroundings. 



It will be unnecessary to remind the reader who 

 knows anything of zoology or of botany that all 

 classification is now based on descent. Since 

 naturalists have abandoned a belief in the special 

 creation of the various species of animals now living 

 on the earth and have conclusively shown that they 

 have arisen by descent and modification from other 

 forms, the problem is to reconstruct a vast genea- 

 logical tree. What then were the ancestors of the 

 fleas and to what other insects, in consequence, do 

 they appear to be related? 



It is probable that the ancestors of the fleas were 

 winged insects, and that the organs of flight were 



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