The Transformation of t lie Mexican Axolotl. 589 



the Tritons as an " adaptation" to a purely aqueous 

 existence." We could here only speak of "adapta- 

 tion " if we took the word in a quite different sense 

 to that in which it was first introduced into 

 science by Darwin and Wallace. These naturalists 

 thereby designate a gradual bodily transformation 

 appearing in the course of generations in corre- 

 spondence with the new requirements of altered 

 conditions of life or, in other words, the action of 

 natural selection, and not the result of a suddenly 

 and direct acting transforming cause exerted but 

 once on a generation. 



Just because the word "adaptation" can be 

 used in ordinary language in many senses, it is 

 desirable that it should have only one precise sig- 

 nification, and above all that we should not speak 

 of adaptation where scarcely any morphological 



Amphibians appear suddenly. The present sexually mature 

 form of the Axolotl has not arisen by the sexual maturity gradu- 

 ally receding in the ontogeny from generation to generation, 

 but by the occurrence of single individuals which were sexually 

 mature in the perennibranchiate stage, these having the advan- 

 tage over the Amblystoma in the struggle for existence under 

 changed climatic conditions. 



By admitting a reversion, we perfectly well explain why arrest 

 at the perennibranchiate stage can be associated with complete 

 development of the sexual organs; the assumption of an 

 " arrested growth " leaves this combination of characters com- 

 pletely unexplained. Moreover, I am of opinion that the 

 expressions " arrested growth " or " reversion " are of but 

 little importance so long as the matter itself is clear.] 



* See Haeckel's " Anthropogenic," p. 449. 

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