94 MERISTIC VARIATION. [part i. 



this number is 15; in Scolopendra it is 21; in Scolopendrops, 23; in 

 Cryptops 21, &c. 



In Geophilidpe, however, the total number of moveable segments is 

 much larger, ranging from about 35 to more than 200. Though not 

 characteristic of genera, the number seems within limits to mark each 

 particular species. It was found that male Geophili have fewer segments 

 than the female. The males of Arthronomalus longicornis have 51 or 

 52 leg-bearing segments, while females usually have 53 or 54. Full- 

 grown females of Geophilus terrestris have 83 or 84 pairs of legs and 

 segments, and the males of the same species have 81 or 82. In a 

 large Neapolitan species, Geophilus Icevigatus Bruhl. 1 the variation 

 is rather greater. In eight males the number varied between 96 

 and 99 ; in eleven females, between 103 and 107. Of two female 

 Geophilus sidcatus one individual had 136 and the other 140. Newport, 

 G., Trans. Linn. Soc, xix. 1845, p. 427, &c. 



[In some of the Chilopoda 1 an increase in the number of segments 

 takes place after the larva hatches, but the variations mentioned above 

 are recorded as occurring in fully formed specimens independently of 

 changes due to age.] 



In the foregoing cases, a fact which is often met in the Study 

 of Variation is well seen. It often happens that in particular 

 genera or in particular species, a considerable range of Meristic 

 Variation is found, while in closely allied forms there is little or 

 none. Examples of this are seen in the variability of the Geophi- 

 lidas as compared with the other Chilopoda, and in the neo-tropical 

 species of Peripatus which vary in the number of legs, while P. 

 balfouri, for instance, is very constant. It will be noticed that in 

 both these cases, the absolute numbers of parts repeated are con- 

 siderably higher in the variable than in the constant forms. But 

 though such cases have given rise to general statements that series 

 of organs containing a small number of members are, as such, less 

 variable than series containing more members, these statements 

 require considerable modification ; for it is not difficult to give 

 instances both in plants and in animals, where series made up of 

 a small number of members, shew great meristic variability. 



The bearings of these cases on the nature of Meristic Repetition 

 and the conception of Homology will be considered hereafter. 

 Here, however, it may be well to call attention to the fact that we 

 have now before us cases in which various but characteristic num- 

 bers of legs or segments differentiate allied species or genera ; that 

 in assuming the truth of the Doctrine of Descent, we have ex- 

 pressed our belief that in each case the species with diverse num- 

 bers are descended from some common ancestor. In the evolution 

 of these forms, therefore, the number has varied : this on the one 

 hand. On the other hand, in Geophilus and in Peripatus, we see 



1 According to Newport (Trans. Linn. Soc. xix. 1845, p. 268), all Myriapoda 

 acquire a periodical addition of segments and legs, but according to later observers 

 this is not true of all the Chilopoda. 



