482 MERISTIC VARIATION. [part i. 



From this some curious results follow. For instance, if two extra 

 limbs arise anteriorly and are separate at their tibial apices, they 

 bear four spurs as shewn at radius A in the upper diagram of Fig. 154. 

 But if the two are fully compounded at the tibial apices in the anterior 

 position the compounded limb will only have two spurs, both being 

 shaped as anterior spurs (as shewn in the lower diagram) and con- 

 versely for the posterior position (see No. 764). The parts, in fact, 

 where the pair may be supposed to interpenetrate (dotted in the diagrams) 

 are not represented. 



Those who have described these phenomena have in consequence 

 often made the following error. Observing a limb giving off' a morpho- 

 logically double limb with a common proximal part subsequently sepa- 

 rating into its two components, they speak of this as a "primary 

 and secondary dichotomy." When the facts are understood it is clear 

 that there is no dichotomy between the extra legs and the normal, for 

 the parts are not equivalent and the normal is undivided. 



Such are the principles followed. It ivould not be true to 

 assert that these rules are followed with mathematical jyrecision, 

 but in the main they hold good. Special attention will be given 

 to cases departing from them, bat the number of such cases is 

 small. The cases of slight deviation from the schematic positions 

 are besides mostly those of extra limbs in the Positions A and P, 

 and generally the deviation in them takes the same form, causing 

 the ventral surfaces of the extra parts to be inclined to each 

 other downwards at an obtuse angle instead of forming one 

 plane. 



In all cases possible I have examined the specimens myself, 

 and I am under obligation to numerous persons who have very 

 generously given me facilities for doing so. Amongst others I 

 am thus greatly indebted to M. H. Gadeau de Kerville, Dr G. 

 Ivraatz and Dr L. von Heyden for the loan of many valuable 

 insects, and also to Messrs Pennetier, Giard, Dale, Mason, West- 

 wood, Waterhouse, Janson, Harrington, Bleuse, &c. In this part 

 of the work I am under especial obligation to Dr D. Sharp, for 

 without his cooperation it would not have been possible for me 

 to have undertaken the manipulations needed. He has most 

 kindly given up his time to the subject, and in the case of almost 

 every one of the specimens examined at Cambridge I have had 

 the benefit of his help and advice. 



Of cases not seen by me few are described in detail sufficient 

 to warrant a statement as to the planes in which the parts stood, 

 but sometimes the figures give indications of this. Some of the 

 accounts are quite worthless, merely recording that such an 

 insect had two extra legs : in such cases I have thought it 

 enough to give the reference and the name of the insect for 

 statistical purposes. But every case known to me is here re- 

 corded : there has been no rejection of cases. 



The cases will be taken in order of the Positions, beginning 



