EVOLUTION 349 



nearly related. Handlirsch has emphasised supposed points 

 of correspondence between the Palaeodictyoptera and the 

 Trilobites — that extinct crustacean group so characteristic 

 of the Palaeozoic seas. Hansen and Lankester have, with 

 more reason, pointed out the close correspondence of the 

 head and its appendages in primitive insects and in the 

 Isopoda, a crustacean order which can be traced back to 

 Devonian times and which has given rise to air-breathing 

 forms like the woodlice. The generally acknowledged 

 relationship of Insects to the Centipedes and the Symphyla 

 leads us, however, to doubt if they can have specially close 

 affinity with so highly organised a crustacean order as the 

 Isopoda. It seems likely that in the origin of insects, in 

 early Palaeozoic times from their primaeval stock, the 

 restriction of their important locomotor limbs to the six 

 thoracic legs must have been a dominant factor in their 

 differentiation as it certainly has been in the subsequent 

 course of their evolution. 



We turn now to consider a large subject which has given, 

 and still gives, occasion for much controversy : the method 

 of evolution in the insect world. This is clearly part of the 

 still larger question of the method of animal evolution 

 generally ; its discussion in a book on the Biology of Insects 

 is appropriate because many lines of inquiry pursued by 

 students of these creatures are found to shed light on those 

 problems of inheritance and development which the student 

 of animal life in its widest aspect seeks to solve. 



The summary of the probable course of insect evolution 

 given in the preceding pages clearly implies the belief that 

 there is a true relationship between the various groups of 

 insects living around us in the world to-day, that these are 

 the descendants of the insects of past ages of the earth's 

 history, and that as the living groups differ in various 

 degrees from one another and from their extinct ancestors, 

 there has been through the long periods of geological time 

 a process of '' descent with modification," a phrase that has 

 often been used to explain and define in brief the modern 



