92 Journal New York Extomological Society. t"^'oi- xxix, 



a type as the biramous mandibular Hmli shown in textfigure i and the 

 greatly modified single segment of such a limb which forms the 

 mandible of the primitive insect shown in textfigure 6. 



Let us for a moment consider the tremendous changes involved in 

 such a leap directly from the type of mandibular limb shown in text- 

 figure I to the type of mandible shown in textfigure 6, without refer- 

 ence to any of the intermediate stages of the series at whose extremes 

 these types stand. The most '' spectacular " change would be the 

 immediate loss of the exopodite "ex" and the endopodite "en'' 

 which are better developed in trilobites than in other arthropods, 

 and represent the extreme of primitiveness as exhibited by the trilo- 

 bite's mandibular limb. The endopodite " en " is relatively larger 

 and better developed in trilobites than is shown in textfigure i (com- 

 pare with correct proportions shown in Fig. 9 of Plate VI) and it 

 still functions as a locomotor appendage in these forms. That such 

 a well developed, perfectly functioning, and apparently useful struc- 

 ture as this endopodite (which is wholly wanting in all insects) 

 should suddenly and completely disappear without first gradually be- 

 coming reduced to fewer segments and losing its importance as a 

 functioning organ of any value (as is shown in the complete series 

 from textfigure i to textfigure 6) is extremely improbable, to say 

 the least ; and it would require more of a mutational leap than even 

 the famous Drosophila has been able to execute, to accomplish " all in 

 one fell swoop " not only the loss of the exopodite and greatly de- 

 veloped endopodite, but the profound modification and intricate dif- 

 ferentiation of the parts of the basal segment which would transform 

 the gnathobase of a trilobite into the highly specialized mandible of 

 an insect ! 



It should be borne in mind that textfigure i is a diagram pure and 

 simple, and consequently the trilobite's gnathobase '' gb " as shown in 

 textfigure i is not as much differentiated as the diagram would indi- 

 cate (since the series shown in the textfigures is intended to empha- 

 size certain points of development thereby making it more readily 

 comprehensible as to how the changes have probably taken place) ; 

 and, as one may see by referring to Fig. 9 (Plate VI) the basal seg- 

 ment of the mandibular limb is still comparatively small in the trilo- 

 bite, and its gnathobase " gb " merely bears a few spine-like pro- 



