NO. 6 INSECT ABDOMEN — SNODGRASS 121 



genital apophyses in form and use, and their legHke relations to the 

 limb bases as opposed to the median, proximal origins of the gona- 

 pophyses. All these points qualify the abdominal styli for true limb 

 rudiments, and give the genital processes the status of basendites 

 specially developed for reproductive purposes. The pretensions of the 

 abdominal styli to telopodite origin are opposed only by their similar- 

 ity to the thoracic styli of MacJiUis; but there is nothing to show 

 that these leg structures are not mere coxal spurs resembling m form 

 but fundamentally unlike the musculated appendages of the abdomen, 

 the styliform shape of which is but one of their many structural 

 adaptations. 



A discussion of the phylogeny of insects, or particularly of the 

 possible origin of insects from any other group of existing arthropods 

 is beyond the intended scope of the present paper. A recent work by 

 Tillyard (1930) on the evolution of the Insecta, though somewhat 

 partisan in favor of myriapodan descent, gives many reasons for 

 believing that insects are not directly related to the Crustacea. And 

 yet, the weight of evidence, whether put forth by claimants of a 

 myriapodan or a crustacean ancestry for insects, seems to depend 

 largely on minimizing or disqualifying the evidence on the other side. 

 However, if we were to give equal weight to arguments on both sides 

 of the question, the insects would be cut off from all ancestral ties, 

 and thereby deprived of a respectable pedigree — unless they are able 

 to take care of themselves through all the unknown ages of time 

 before they are first known to us as fully-winged hexapods in the 

 Carboniferous deposits. To the writer it appears that all the principal 

 arthropod groups must represent independent lines of descent from 

 some remote ancestral forms embodying the potentialities of a spider, 

 a crab, a centipede, or an insect. It has recently been emphasized 

 by Clark ( 1930) that the chronic inability of the evolutionary theory 

 in its usual form to explain the lack of intermediates between the 

 major groups of animals constitutes a real weakness of the theory, 

 which calls for a new concept of the method by which distinct types 

 of organisms have been produced. The condition to which Clark 

 refers is well exemplified within the Arthropoda, where connective 

 forms between the classes are unknown. Moreover, it is impossible 

 to construct imaginary arthropods that will fill the blanks, as, for 

 example, the three-cornered gap between the crustaceans, the myria- 

 pods, and the insects. Considering that embryos develop before 

 our eyes by ways that are still inscrutable, it takes a strong faith in 

 established ideas to believe that organic evolution has proceeded 

 entirely by the means we have furnished for its guidance. 



