59 S Studies in the Theory of Descent. 



is exactly the same as between Triton and its 

 " sexually mature larva;" the difference between 

 the two cases of reversion depends entirely upon 

 the direction of the leap, that taken in the former 

 case being precisely opposite in direction to that 

 taken in the latter. 



Herein might be sought the explanation of the 

 different strength with which the reproductive 

 power is affected ; not indeed in the direction of 

 the leap itself, but in the differences in the ontogeny 

 which are determined by the differences in the 

 direction of the leap. The reversion of the 

 Triton to an older phyletic stage coincides with 

 the arrest at a younger ontogenetic stage ; or, in 

 other words, the older stage of the phylogeny to 

 which reversion takes place is still entirely 

 comprised in the ontogeny of each individual. 

 Each Triton is perennibranchiate throughout a 

 long period of its life ; the reverting individual 

 simply reverts to the older phyletic stage by 

 remaining at the larval stage of its individual 

 development. 



But it is quite different with the reversion of the 

 Axolotl to the formerly acquired, but long since 

 abandoned Amblystoma form. This is not retained 

 in the ontogeny of Axolotl, but has been com- 

 pletely lo.^t ; for a long series of generations so 

 must we suppose the ontogeny has always only 

 attained to the perennibranchiate form. Now if at 

 the present time certain individuals were compelled 



