CLASSIFICATION OF INSECTS. 53 



The existing Thysanurans are nevertheless quite 

 widely removed from the ancestral wingless form, with 

 which their own type probably came into existence, 

 and they possess speciahzations of their own which 

 should be taken into account. The scales and charac- 

 teristic mouth parts of Lepisma are probably speciali- 

 zations acquired by this genus, while the small pro- 

 thorax and very distinct head of Campodea are also 

 specialized characteristics. The larvae of the general- 

 ized forms of insects also have peculiarities in their 

 internal structures and mouth parts which tell the 

 same story of acquired specializations. We mean by 

 the "generalized orders of insects " what we shall also 

 call the first series of orders, those numbered from I. 

 to IX., inclusive, which may be considered as a whole, 

 and thus more advantageously compared with the 

 specialized orders of insects, or the orders numbered 



regain the organ, although they may develop a substitute for it. 

 The group of Geometrina, a number of whose larval forms have 

 lost two or three pairs of abdominal legs, cannot, according to 

 this view, give rise to a genus which will recover the lost pairs 

 of legs, and therefore must be regarded as a terminal develop- 

 ment or a group which ends in itself. 



Although this view may be considered as unproven in its 

 general application and as difficult to demonstrate among in- 

 sects, it helps us to show that, as a rule, it is more natural to 

 place the parasites and other types that have suffered through 

 specialization by reduction at the head or termini of their re- 

 spective groups rather than to follow the prevalent fashion of 

 introducing them at the beginning of classifications. They repre- 

 sent, in other words, the finished work of evolutionary processes 

 acting through periods of time more or less prolonged, and not 

 the beginning or first steps in the evolution of their own groups 

 and are not in any sense primitive or generalized. 



