CHAP. XV.] 



tarsus of Blatta. 



417 



To this question a positive answer cannot yet be given ; but as about 

 200 young B. orientalis have since been hatched from the egg and no 

 4-jointed tarsus was found among them, while in every instance of 

 regeneration the new tarsus had four perfect joints, there is now a 

 presumption that the variation does not occur congenitally. On the 

 other hand it should be mentioned that the 4-jointed tarsus was seen 

 in 3 specimens, found by Mr Brindley, which by their size would be 

 judged to have been newly hatched. But even if the variation shall 

 hereafter be found to be sometimes congenital it is certain that this 

 occurrence must be very rare, and there can be no doubt that in the 

 majority of cases the 4-jointed tarsus has arisen on regeneration 1 . 



As mentioned in the Introduction, the existence of the 4-jointed 

 tarsus, whatever be the manner of its origin, raises two questions. Of 

 these the first is morphological, relating to the degree to which the 

 joints exhibit the property of individuality, and the second is of a 

 more general nature, relating to the application of the theory of 

 Natural Selection to such a case of discontinuous change. The interest 

 of the case in its bearing on both of these questions arises from the 

 Discontinuity, which was complete. All the tarsi seen were either 

 5-jointed or 4-jointed, and in none of the latter was any joint ever 

 rudimentary, or any line of articulation imperfectly formed (except 

 in a single specimen having a deformed tarsus). There were 5-jointed 

 tarsi and 4-jointed tarsi : between them nothing. 



Following the usual methods of Comparative Anatomy it must be 

 asked which of the 5 joints is missing in the 4-jointed tarsus'? With 

 reference to this question careful measurements of the separate joints 

 were made by Mr Brindley in 115 cases of 4-jointed tarsi occurring in 

 legs of the third pair in B. americana; and for comparison the separate 

 joints of 115 normal 5-jointed third tarsi of the same species were also 

 measured. (It is clear that the legs compared must belong to the same 

 pair, 1st, 2nd or 3rd, for there is considerable differentiation between 

 them. From this circumstance it was comparatively difficult to obtain 

 a large number of cases, and hence the smallness of the whole number 

 measured. But though of course statistics respecting a larger number 

 would be more satisfactory there is no reason to think that by exami- 

 nation of a greater number of cases the result would be materially 

 affected.) 



In the two sets of tarsi the total length of each tarsus was reduced to 1-000, the 

 lengths of the joints being correspondingly reduced. 



The arithmetic means of the ratios of the several joints to the whole lengths of 

 the tarsi to which they belonged was as follows : 



Five-jointed form. 



1st joint 

 •532 



2nd joint 

 •156 



3rd joint 

 •095 



4th joint 

 •019 



5th joint 

 ■168 



Four-jointed form. 



1st joint 

 •574 



2nd joint 

 •183 



3rd joint 

 •061 



4th joint 

 •179 



1 The circumstance that in Mr Brindley's observations the variation was in all 

 species more frequent in females than in males, and that the frequency differed in 



b. 27 



