REPTILES AND AMPHIBIANS 145 



of regeneration in various parts of animals that have 

 otherwise a high power of regenerating, its origin as 

 an adaptive phenomenon becomes perfectly clear. The 

 newts that re-form eyes and legs cannot regenerate 

 internal organs. If we cut the lungs out of a triton, 

 or the ovary or any other internal organ, and sew it 

 together again, there is no regeneration of the part in 

 question. Why ? Because such a mutilation is hardly 

 possible in nature, and so could not be provided against. 

 While the newt has this admirable power of regenera- 

 tion to protect him against his enemies, the frog owes 

 its maintenance generally to its enormous fertility. 

 Their eggs, the familiar frog-spawn, are not only 

 poured into the water in large numbers, but are also 

 provided with a peculiar protective structure, which 

 consists of a jelly surrounding the egg. This jelly 

 prevents the eggs from being dried up or crushed, and 

 especially protects them from enemies. Birds, fishes, 

 crustaceans, and other animals are unable to eat the 

 eggs, because they slip out of the mouth again. Every- 

 one knows how difficult it is to handle frog-spawn. 

 The transparent balls of jelly also act as lenses for 

 focussing the rays of light, and thus attract movable 

 plants, such as certain algae, that produce oxygen, 

 which is good for the eggs. The jelly also lets the 

 rays of light pass direct to the eggs, and keeps them 

 there, so that they are practically surrounded by glass- 

 houses. The spawn is slightly heavier than water, and 

 if a high temperature causes an increased outflow of 

 gas from the plants, the bubbles collect on the spawn, 

 and bring it to the surface, where it receives the full 



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