240 THE HISTORY 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 
