1 6 METAMORPHOSES OF TADPOLES 



chiefly to bones and muscles. But enlargement with 

 increased physical strength, and diminution of size 

 with impaired strength, are quite different things 

 from transmutation. 



Organs are used because they are fit for use ; 

 organs are not used because they are unfit for use. 

 We can, for example, observe how the limbs and 

 lungs of the frog come into use, and how the tail 

 and gills of the tadpole fall into disuse ; and how 

 the animal from being virtually a fish comes thereby 

 to be an air-breathing Batrachian. 



The limbs of the young frog thus metamorphosed 

 come into use because they have come into existence 

 under all the conditions fitting them and exciting 

 them to perform their appropriate actions. The 

 animal in the tadpole state has no need of the limbs 

 before they are formed ; and when they have been 

 formed the animal must necessarily use them, 

 because the conditions for their action have come 

 into existence along with themselves. 



Let us now see how an organ falls out of use and 

 disappears. This process we can observe in the tail 

 of the tadpole. That organ does not disappear 

 because it ceases to be used, but ceases to be used in 

 proportion as it shrinks and disappears ; while the 

 fore legs of the future frog become, like its already 

 developed hind legs, free and fit for action. 



These changes constitute a stage in the natural 

 process of development to the perfect frog-state, when 

 ^he respiration by gills is at the same time superseded 



