240 THE HISTOHY OF CREATION. 



tinual practice. Compare, for example, the arms and legs 

 of a trained gymnast with those of an immovable book- 

 worm. 



How powerfully external influences affect the habits of 

 animals and their mode of life, and in this way still further 

 change their forms, is very strikingly shown in many cases 

 among amphibious animals and reptiles. Our commonest 

 indigenous snake, the ringed snake, lays eggs which require 

 three weeks' time to develop. But when it is kept in 

 captivity, and no sand is strewn in the cage, it does not lay 

 its eggs, but retains them until the young ones are developed. 

 The difference between animals producing living offspring 

 and those laying eggs is here effaced simply by the change 

 of the ground upon which the animal lives. 



The water-salamanders, or tritons, which have been 

 artificially made to retain their original gills, are extremely 

 interesting in this respect. The tritons are amphibious 

 animals, nearly akin to frogs, and possess, like the latter, 

 in their youth external organs of respiration — ^gills — with 

 which they, while living in water, breathe the air dissolved 

 in the water. At a later date a metamorphosis takes place 

 in tritons, as in frogs. They leave the water, lose their gills, 

 and accustom themselves to breathe with their lungs. But 

 if they are prevented from doing this by being kept shut up 

 in a tank, they do not lose their gills. The gills remain, and 

 the water salamander continues through life in that low 

 stage of development, beyond which its lower relations, the 

 gilled salamanders, or Sozobranchiata, never pass. The gilled 

 salamander attains its full size, its sexual development, and 

 reproduces itself without losing its gills. 



Great interest was caused a short time ago, among 



