II THE AXOLOTL 45 



separation of species, to the just-mentioned action of correla- 

 tion, to what I have called intermittent evolution. 



Intermittent Evolution — Kolliker's Hypothesis 



It appears, therefore, according to my views, to be pos- 

 sible, that since one character is followed by one or perhaps 

 several others correlated with it, a new form may suddenly 

 arise, which, if the variation extends to the sexual products, 

 may be no longer capable of crossing with the original form. 

 At the same time, this latter condition is not necessary for the 

 production of a new species. For example, hair, horns, and 

 hoofs have been proved to be connected correlatively with 

 one another, and by simultaneous variations in these three 

 organ -systems a very considerable change in the external 

 appearance of an animal may be effected. If only one of the 

 altered systems is more useful to the animal than that of the 

 original form, then, notwithstanding the possibility of crossing, 

 a new species may arise. 



There are of course few examples among living animals 

 which illustrate this mode of origin in so complete a manner 

 as the Axolotl. But if this were the only one it would 

 suffice, and would justify the conclusion that the same meta- 

 morphosis has under similar conditions probably occurred in 

 other allied animals. The metamorphosis of the Axolotl into 

 Amblystoma is clearly the consequence of the transition of 

 the animal from an aquatic to a terrestrial existence. With 

 the cessation of branchial and the commencement of ex- 

 clusively pulmonary respiration went correlatively hand in 

 hand the appearance of a spotted marking on the skin, of the 

 smoothness of the latter in place of the warty character, 

 especially on the head, of that of the Axolotl, the slight change 

 in the form of the body, and lastly the formation of a more 

 cylindrical tail, while also, in consequence of terrestrial 

 habits, the legs became more powerful. It is highly 



