126 MERISTIC VARIATION. [part i. 



of several cases already given, it is not impossible that if the 

 variation seen in this frog had gone further, the 10th vertebra 

 might alone support the ilium (cp. Nos. 57 and 60) and thus 

 present the characters of the normal 9th in their completeness. If 

 this change had taken place, we should have a case like that of 

 Gavialis, and there would be nothing to shew that the new 10th 

 vertebra was not the 9th of the normal. The truth then seems to 

 be that owing to the correlation between Meristic Variation pro- 

 ducing change in number, and simultaneous Substantive Variation 

 producing a change of form or rather a redistribution of characters, 

 the attempt to trace individual homologies must necessarily fail ; 

 for while such determination must be based either on ordinal 

 position or on structural differentiation, neither of these criterions 

 are really sound. As I have tried to shew, the belief that they are 

 so depends rather on preconception than on the facts of Variation. 



*57. A male specimen of B. temporaria <£ with ten free vertebrae is 

 described by Howes. In this case the 9th had a posterior 

 zygapophysis on the left side only. Upon the left side the 

 transverse process of the 9th was not larger than that of the 8th 

 and did not support the ilium, which on the left side was entirely 

 borne by the large transverse process of the 10th. On the right 

 side the transverse processes of both 9th and 10th were developed 

 to support the ilium, neither being in itself so large as that of the 

 10th on the left side. The 9th was concave in front instead of 

 convex as usual, and thus the 8th which is normally biconcave is 

 convex behind. The posterior faces of both 9th and 10th bore two 

 convexities such as are normal to the 9th. The urostyle was 

 normal, having well-developed apertures for exit of the last pair 

 of spinal nerves. Howes, G. B., Anat. Anz., I. 1886, p. 277, 

 figures. 



In this case the departure from the normal, exemplified by 

 No. 56, has gone still further, and the new 10th vertebra bears the 

 ilium wholly on the left side and in part on the right. The con- 

 dition is thus again intermediate between the normal and a com- 

 plete transformation of the 9th into a trunk vertebra and the 

 introduction of a 10th to bear the ilium (as in No. 60). As 

 regards the homologies of the vertebra?, the same issues are 

 atjain raised which were indicated in regard to No. 56. 



58, Rana temporaria : Case in which transverse processes were present 

 in the atlas vertebra and the transverse processes of several of the 

 vertebrae were abnormal (Fig. 11, I.). The atlas possessed well- 

 developed transverse processes. 



In the axis the transverse processes are directed forwards instead 

 of backwards, and that of the left side presents an indication of bifur- 

 cation at its extremity. 



The third vertebra possessed two pairs of transverse processes 

 which are joined together for two-thirds of their length. The fourth 



