L82 MKIMsTir VARIATION. [PARTI. 



tlii- -.in,, curious re-ult- i'n]].i\v. For instance, if two extra 

 liinl.- arise anteriorly and an- separate at their tibial apices, they 

 bear four -purs as shewn at radius A in the upper diagram of Fig. 1 > 1. 

 I '.ut it' tip- two an- fully compounded at the tibial apices in the anterior 

 po-itioii tin- ronipMiiinlcil limb will only have two spurs, both being 

 -haped ae anterior -pur- (as shewn in the lower diagram) and con- 

 versely for tin- posterior position (see No. 7'i4). The parts, in fact, 

 win- re the pair may be supposed to interpeiiet rate ,, lotted in the diagrams) 

 are not represented. 



Tho-e u ho ha\e described these phenomena have in consequence 

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

 logically double limb with a common proximal part subsequently sepa- 



rating into it- t \\ o components, they speak of this as a "primary 

 and -econdary dichotomy When the facts are understood it is clear 

 that there j- nit dichotomy between the extra legs and the normal, for 

 the parts are not ci|iii\alent and the normal is undivided. 



Such are tin- principles followed. It would not be true to 

 ..... // tlnif iln'sf rules wre followed with mathematical />recixi/i, 

 hut in tin- ni'i/ii tin-;/ Lnlii ,/, ,:n/. Special attention will be given 

 to cases departing from them. but the number of such cases is 

 -mall. The cases of slight deviation from the schematic positions 

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

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

 tin- ventral sin-faces of the extra parts to be inclined to each 

 other downwards at an obtuse angle instead of forming one 

 plane. 



In all cased possible I have examined the specimens myself, 

 and I am tinder obligation to numerous persons who have ver\ 

 generously given me facilities tor doing so. Amongst others 1 

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

 Krait/. and Dr L. von Heyden f>r the loan of many valuable 

 insects, and also bo Messrs IVnnetier, (Hard, Dale, Mason, West- 

 n-ood. YVaterhoiise, .laiison, Harrington, I'.leuse, Me. In this part 

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

 \\ithoiit his i ..... pei-ation 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 al Cambridge I have had 

 tin- benefit of his help and advice. 



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

 to warrant a state ..... nt as to the planes in which the parts stood, 

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

 accounts are ipiite 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 f the insect for 

 statistical purposes. |; u t every case known to me is here re- 

 corded: tin-re has been no rejection of cases. 



The cases \\ill be taken in order of the Positions, beginning 



