SECT, xi.] INTRODUCTION. 63 



this new number may prevail in some or in all of those systems of 

 organs which are disposed around the common centre. 



2. Linear Series. 



Before speaking further of the totality or perfection of Varia- 

 tion it will be well to give an illustration of Discontinuous Meristic 

 Variation as it occurs in the case of a linear series of parts. As 

 such an illustration the case of the variation in the number of 

 joints in the tarsus of the Cockroach (Blatta) may be taken. This 

 variation has been the subject of very full investigation by Mi- 

 ll. H. Brinclley. The tarsus of the Cockroach is normally divided 

 into five joints, but in about 25 per cent, of B. americana (and in 

 a smaller proportion of several other species) the tarsus of one or 

 more legs is divided into only four joints, though the total length 

 may be the same as that of the corresponding leg of the other side, 

 Fig. 8. Between the five-jointed form of tarsus and the four-jointed 

 form no single case in any way intermediate was seen. The whole 



n 



FIG. 8. Tarsi of the third pair of legs in a specimen of Blatta americana, 

 I. the left tarsus, having the normal, or 5-jointed form ; II. the right tarsus, 

 having the 4-jointed form. 



evidence will be given in full in the proper place and raises many 

 questions of great interest ; but that which is important to our 

 present consideration is the fact that the Variation is here un- 

 doubtedly discontinuous, arising suddenly as a total or perfect 

 Variation, from the five-jointed form to the four-jointed. Here 

 the variation, though total as regards the limb in which it is 

 present, is not total as regards all the legs taken together. For 

 commonly only a single leg had a four-jointed tarsus, and only one 

 specimen was met with in which all six legs thus varied, and one 

 specimen only shewed the variation in five legs. 



In speaking of such a Variation as a perfect Variation several 

 things are meant. 



First, it is meant that the tarsus of the new pattern is as 

 distinctly divided into four joints as the normal is into five. In 



