THE GILLS OF SALAMANDERS. 241 
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 
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