46 THE ORGANIC GROWTH OF THE LIVING WORLD sec. 



remarkable that the same peculiarities distinguish Sala- 

 mandra maculata in comparison with its aquatic larva. 

 In fact, it is scarcely possible to distinguish a Salamander 

 larva so long as it retains its gills from a young Siredon, 

 Both have the gray colour without large yellow -spots, 

 the laterally compressed swimming tail, and the feeble legs ; 

 and lastly, in comparison with the adult Salamander or 

 Amblystoma, respectively, a somewhat flatter, longer, and 

 narrower head, with less prominent eyes. 



Siredon is for the problem of evolution one of the most 

 important of living animals, in that it brings so beautifully 

 before our eyes the transition of a lower sexually mature 

 into a higher sexually mature form, and at the same time 

 shows so clearly the causes of the transformation. We 

 discern these causes simply in the reaction of the organism 

 under external conditions, the increased exertion of an 

 organ already in process of formation (the lung), and the 

 disappearance of another (gills) in consequence of definite 

 demands of the environment, and changes connected with 

 these by correlation.^ 



Amblystoma appears where the Axolotl has too little water 

 to live in, where it is compelled to live on dry land. That this 

 is the case is proved by the possibility of artificially rearing 

 Amblystoma from the Axololt by gradually withdrawing the 

 water. Before this can be possible it must of course be a 

 previous condition that the Axolotl has already a great 

 tendency to transform itself into a land animal. This may 

 be due to the possible fact that it has in its aquatic state 

 already made much use of its lungs ; but the transformation 

 which follows its assumption of terrestrial habits is none the 

 less sudden, and evidently connected with correlative changes. 

 When an Amblystoma is formed, then this is the adult animal, 



^ These are the essential causes ; I do not pretend to have exhausted the 

 explanation of the metamorphosis. 



