in] OTHER DEFINITIONS 77 



All goes well up to the Pilidium, but then comes the 

 gap. There is no case known where two complete 

 individuals are formed as the result of metamorphosis. 

 In reality, the detachment of the Pilidium skin from 

 the young worm is not an attempt at reproduction at 

 all, but is due to something very different. This 

 something is the incompleteness of adaptability in 

 protoplasm, and since the subject will concern us 

 again later (p. 132 et seqq.) it may be investigated here. 



The whole raison d'etre of a metamorphosis is the 

 restriction of the animal to one environment in one 

 period of its life, to another and a wholly different 

 environment in another period. Different environ- 

 ments require different structures ; and the metamor- 

 phosis is the time when the old structures are destroyed. 

 When the tadpole, for instance, suffers a land-change, 

 gills and tail must vanish. They do not, like the 

 skeleton of the gills, become converted, after con- 

 siderable remodelling, into structures of the adult, 

 nor like the caterpillar's outer skin, are they bodily 

 cast off: they are absorbed, they shrink and their 

 contents are drawn into the body of the young frog 

 for future use, as the yolk-sac and its contents are 

 drawn into the body of the unhatched chick. 



The tadpole is so well adapted to the water, the 

 frog so well adapted to the land, that certain organs 

 cannot be used, however remodelled, for life on both. 

 They cease to exist as such ; it is only the materials 



