Tune. i9::i.l CrAMPTOX : EvOLL'TIOX OF THE AxTHROPODA. 65 



brian times, enables us readily to compare the more recently evolved 

 forms with their " contemporaneous ancestors."' and it is therefore 

 frequently quite easy to obtain an almost unbroken series of stages 

 illustrating the probable steps in the evolution of certain structures 

 almost from the very inception of the development of the tendency on 

 the part of these structures to assume their more modified aspects, 

 "^ince I have been fortunate in obtaining an exceptionally fine series 

 illustrating the probable course of the phylogenetic development of 

 one type of insectan mandible, it has seemed preferable to make this 

 the subject of the first paper dealing with the evolution of arthropods 

 related to insects. 



Since no living types are strictly speaking " ancestral " to other 

 living types (excepting in the case of mutants which have departed 

 but little from the parent stock), it should be clearly understood that 

 in employing a number of recent forms to illustrate the path of 

 evolution followed in deriving the insectan type of mandible from 

 the original arthropodan type of mandibular appendage. I would not 

 imply that any one of the stages represented in the series is actually 

 ancestral to the succeeding stages. On the other hand, certain primi- 

 tive living forms have departed but little from the actual ancestors 

 of other living forms in many respects, and those "' ancestral "' 

 features which they have preserved in a very slightly modified form, . 

 -erve to indicate the probable stages through which the parts of other 

 more highly modified forms have passed, in assuming their present 

 condition; and the study of such a series is of the greatest value in 

 enabling us to gain a correct understanding of the nature of the 

 parts in the higher forms. 



As a rule, the student of trilobitan structures has confined his 

 attention to this group alone, and the carcinologist is content to 

 levote his energies to the study of the Crustacea alone, while those 

 entomologists who have attempted to invade these fields have not been 

 conspicuously successful in comparing the structures of insects with 

 those of Crustacea and trilobites, with the result that the true nature 

 of the parts of insects is not understood in many cases, and the most 

 glaring misinterpretations of insectan structures have gained a dis- 

 hearteningly widespread acceptance in the various te.xtbooks and 

 publications dealing with this phase of entomology. In this con- 



