THE GILLS OF SALAMANDERS. 24 1 



zoologists, by the axolotel (Siredon pisciformis), a gilled 

 salamander from Mexico, nearly related to the triton ; it 

 had already been known for a long time, and been bred on a 

 large scale in the zoological garden in Paris. This animal 

 possesses external gills, like the young salamander, but 

 retains them all its life, like all other Sozobranchiata. This 

 gilled salamander generally remains in the water, with its 

 aquatic organs of respiration, and also propagates itself 

 there. But in the Paris garden, unexpectedly from among 

 hundreds of these animals, a small number crept out of 

 the water on to the dry land, lost their gills, and changed 

 themselves into gill-less salamanders, which are not to be 

 distinguished from a North-American genus of tritons 

 (Amblystoma), and breathe only through lungs. In this 

 exceedingly curious case we can directly follow the great 

 stride from water-breathing to air-breathing animals, a 

 stride which can indeed be observed every spring in the 

 individual history of development of frogs and salamanders. 

 Just as every separate frog and every separate salamander 

 transforms itself from an amphibious animal breathing 

 through gills, at a later period into one breathing through 

 lungs, so the whole group of frogs and salamanders have 

 arisen from animals breathing through gills, and akin to the 

 Siredon, The Sozobranchiata have remained up to the 

 present day in that low stage of development. Ontogeny 

 here explains phylogeny ; the history of the development 

 of individuals explains that of the whole group (p. 10). 



To the law of accumulative adaptation there closely fol- 

 lows a third law of direct or actual adaptation, the law of 

 correlative adaptation. According to this important law, 

 actual adaptation not only changes those parts of the 



