ON CHANGE OF MEXICAN AXOLOTL TO AN AMBLYSTOMA. 367 



larvae that laid eggs exactly prove that even in reversion the power of 

 reproduction may remain complete. From the universal causes of ster- 

 ility mentioned above, it mpy be inferred that through those causes 

 frnitfalness may be lost in different degree: and, further, it may in a 

 measure be understood why the fruitfulness is more completely lost in 

 the reversion to the Amblystoma form than in the reversion of the Tri- 

 ton to the Perennibranchiate form. 



If iu these cases the reversion is called forth by altering the con- 

 ditions of life, one may perhaps imagine that the extent of this altera- 

 tion will also fix the degree of fertility which the atavistic form can pre- 

 serve; but still more will the fertility be influenced by the extent of the 

 morphological sTcip which is made in the reversion. 



We know that the blending of widely varying constitutions (e. g., in 

 the crossing of different species) produces sterility. Something similar 

 takes place also iu the sudden reversion to a stage of development widely 

 varying in its whole structure. Here also takes place to a certain extent 

 the union of two very different constitutions in one individual, a sort of 

 crossing. 



From this point of view it may be, to some extent, understood why 

 sterility may be a result of the reversion, but, on the other hand, we ob- 

 tain no explanation why with the same breadth of morphological varia- 

 tion there occurs in one case complete sterility, in the other relative 

 fruitfulness. The extent of the morphological contrast is exactly the 

 same between Axolotl and Amblystoma as between Triton and its 

 '^sexually mature larvae." The difference in the two cases of reversion 

 lies entirely in the direction of the skip, which in the first case is made 

 exactly in the contrary direction to that in the second. 



Just in that I would seek the reason for the varying force with 

 which the power of reproduction is affected ; not in the direction of the 

 leap in and for itself but in the differences of the ontogenesis, which 

 are conditioned upon the direction of the leap. 



The reversion of the Triton to o,n older phyletic stage coincides with 

 remaining upon a younger ontogenetic stage ; or, in other words, the older 

 stage of phylogenesis^ to which the reversion talces place, is still entirely in- 

 cluded in the ontogenesis of every individual. Every Triton is a Perenni- 

 branchiate for a considerable portion of its life; the individual that 

 makes the reversion simply reverts to an older phyletic stage by re- 

 maining in the larva stage of its individual development. 



Quite otherwise with the reversion of the Axolotl to the Amblystoma 

 form, which had been once reached at some former time, but long since 

 given up. This is not included in the ontogenesis of the Axolotl, but has 

 entirely fallen out. For a long succession of generations — so we must 

 suppose — the ontogenesis has always proceeded only to the Perenni- 

 branchiate form. If now single individuals are made to revert to the 

 Amblystoma form, no greater leap in respect to morphology is made 

 than in the reversion of the Triton to the Perennibranchiate form. At 



