The Transformation of the Mexican Axolotl. 597 



general causes of sterility, it may even be inferred 

 that fertility can be lost in different degrees, and it 

 can be further understood to a certain extent why 

 this fertility is more completely lost by reversion 

 to the Amblystoma, than by the reversion of the 

 Triton to the perennibranchiate form. 



If in these cases the reversion is brought about 

 by a change in the conditions of life, we may 

 perhaps suppose that the magnitude of this change 

 would determine the degree of fertility, and the 

 preservation of the reversion form. Still more, 

 however, would the fertility be influenced by the 

 extent of the morphological difference resulting 

 from the reversion. We know that the blending 

 of very different constitutions (e. g. the crossing of 

 different species) produces sterility. Something 

 similar results from the sudden reversion to a stage 

 of development widely different in its whole 

 structure. Here also we have in a certain sense 

 the union of two very different constitutions in one 

 individual a kind of crossing. 



From this point of view it can in some measure 

 be comprehended why sterility may be a result of 

 reversion ; on the other hand, we thereby obtain no 

 explanation why, with the same amount of morpho- 

 logical difference, in one case complete sterility, and 

 in another relative fertility occurs. The morpho- 

 logical difference between Axolotl and Amblystoma 



(Cat. tridentatum) and this is always sterile. Darwin, " Fer- 

 tilization of Orchids," and ed. p. 199.] 



