186 THE GENESIS OF SPECIES. [CHAP. 



blance existing between parts which are placed one above, 

 the other beneath.. It is much less general and marked 

 than serial, or lateral homology. Nevertheless, it is plainly 

 to be seen in the tail region of most fishes, and in the far- 

 extending dorsal and ventral fins of such kinds as. the sole 

 and the flounder. 



It is also strikingly shown in the bones of the tail of 

 certain efts, as in Chioglossa, where the complexity of the 

 upper (neural) arch is closely repeated by the inferior one. 

 Again, in Spelerpes rubra, where almost vertically ascend- 

 ing articular processes above are repeated by 

 almost vertically descending articular processes 

 below. Also in the axolotl, where there are 

 double pits, placed side by side, not only supe- 

 riorly, but at the same time inferiorly. 1 



This kind of homology is also explained by 

 Mr. Spencer as the result of the similarity of 

 conditions affecting the two parts. He explains 

 the very general absence of symmetry between 

 the dorsal and ventral surfaces of animals by 

 OF AXOLOTL. the different conditions to which these two sur- 

 faces are respectively exposed, and in the same way he 

 explains the asymmetry of the flat-fishes (Pleuronectidce), 

 of snails, &c. 



Now, first, as regards Mr. Spencer's explanation of 

 animal forms by means of the influence of external condi- 

 tions, the following observations may be made. Abundant 

 instances are brought forward by him of admirable adapta- 

 tion of structure to circumstances, but in the immense 

 majority of these instances it is very difficult, if not impos- 



1 See a Paper 011 the "Axial Skeleton of the Urodela," in Proc. Zool. 

 Soc. 1870, p. 266. 





