236 



EVOLUTION AND ANIMAL LIFE 



or chrysalid stage, the changes seem to be made by sudden 

 leaps. Of course, this is not so. It is all done gradually, 

 although there are certain periods in the course of the develop- 

 ment when the changing is more rapid and radical than at 

 other times. The changing is masked by the outer covering 

 of larva and pupa, and although it is indeed startlingly radical 

 in its character, it is wholly continuous. 



The metamorphosis of frogs and toads also is familiar. 



FIG. 139. Stages in development of silkworm moth. 



The eggs of the toad are arranged in long strings or ribbons 

 in a transparent jellylike substance. These jelly ribbons 

 with the small, black, beadlike eggs in them are wound around 

 the stems of submerged plants or sticks near the shores of 

 the pond. From each egg hatches a tiny, wriggling tad- 

 pole, differing nearly as much from a full-grown toad as a 

 caterpillar differs from a butterfly. The tadpoles feed on the 

 microscopic plants to be found in the water, and swim easily 

 about by means of their long tails. The very young tadpoles 

 remain underneath the surface of the water all the time, breath- 

 ing the air, which is mixed with the water, by means of gills. 

 But as they become older and larger they come often to the 

 surface of the water. Lungs are developing inside the body, 

 and the tadpole is beginning to breathe as a land animal, 

 although it still breathes partly by means of gills, that is, as 



