582 Studies in the Theory of Descent. 



often very extensive change or transformation, 

 only apparently takes place suddenly, and is but 

 the apparent result of the action of cold upon this 

 generation, whilst in fact it depends upon reversion 

 to the primary form of the species, so that the 

 low temperature, which is only once applied, gives 

 but the impetus to reversion, and is not the true 

 cause of the transformation. This cause must 

 rather be sought in the long continued action of 

 the cold to which the ancestors of our existing 

 butterflies were subjected for thousands of genera- 

 tions, and of which the final result is the winter 

 form. 



If we assume for an instant that my interpreta- 

 tion of the transformation of Axolotl as just offered 

 is correct, we should have conditions in many 

 respects analagous to those of seasonal dimor- 

 phism. It is true that in this case the two forms 

 no longer alternate regularly with each other, but 

 the primary form may occasionally appear instead 

 of the secondary form, owing to the action of 

 external conditions. 



Just as in the case of seasonal dimorphism it 

 is possible to compel the summer generation to 

 abandon the summer form, and to assume the 

 winter guise by the action of cold ; so in the 

 present case we are able to induce the Axolotl to 

 adopt the Amblystoma form by making aerial 

 respiration compulsory at a certain stage of life ; 

 and further, just as in seasonal dimorphism it can 



