VIII.] HOMOLOGIES. 179 



" primitive groove." This kind of homology is explained 

 by Mr. Spencer as the result of the similar way in which 

 conditions affect the right and left sides respectively. 



Vertical homology (or vertical symmetry) is the resem- 

 blance existing between parts which are placed one above 

 the other bt^neath. It is much h^ss 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 (back) and ventral (belly) fins of such kinds 

 as the sole and the flounder. 



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

 certjiin efts, as in Chiofflossa^ where the complexity of the 

 upper (neural) arch is closely repealed by the infc_ 

 rior one. Again, in Spderpes rubra, where almost 

 vertically ascending 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 su- 

 periorly but at the same time inferiorly.'' 



This kind of homology is also explained by 

 Mr. Spencer as the result of the similarity of con- 

 ditions affecting the two parts. Thus he explains 

 the very general absence of symmetry between the "^^■^^"** 

 dorsal and ventral surfaces of animals by the differ- ^^^'^^'^ 

 ent conditions to which these two surfaces are respectively 

 exposed, and in the same way he explains the asymmetry 

 of the flat fishes (Plei(ronectidai), of snails, etc. 



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

 forms by means of the influence of external conditions, the 

 following observations may be made : Abundant instances 

 are brought forward by him of admirable adaptation of 

 structure to circumstances, but as to the immense major- 



' Sec a Paper on the " Axial Skeleton of the Urodela," in Proc. ZooL 

 Soc, 1870, p. 266. 



