AND THE LOWER ANIMALS. 121 



Look at the gradual disappearance of the gills and 

 tail of a tadpole, and of the rotary apparatus of 

 the Teredo, and although you be not a naturalist, 

 you will say, " These are organs which have atrophied" 

 Compare the young crab's tail with that of the adult ; 

 the reproductive organs of a neuter with those of a 

 queen bee ; and you will yourself employ the expres- 

 sion — arrest of development. 



Observe the feet of the Lernea, at first employed as 

 oars, and afterwards, when converted into a sort of 

 anchor, serving to attach the animal to its living 

 residence, and you will be struck with the manner in 

 which Nature adapts an already existing organ to an 

 entirely new function. Tou will then perceive that 

 metamorphosis, properly so called, is above all things 

 characterized by transformations. 



In order that these phenomena shall take place, it 

 is necessary that the same stream of matter shall 

 flow to and from the parts as that which exists in 

 Mammalia. In the great majority of cases the altera- 

 tions do not occur suddenly, and they take place, 

 moreover, in the very depth of the tissues. The 

 branchiae of the tadpole do not fall off directly to give 

 place to the lung ; nor does the tail become detached 

 when the limbs are formed. No; in proportion as 

 the one is developed, with its bones, muscles, nerves, 

 and vessels, the other decreases in every portion of 

 its structure and in all its parts simultaneously. The 

 latter is actually resorbed molecule by molecule, just 

 as the former is being built up atom by atom. 



It is true that in Insects and Crustacea there is 

 apparently a different state of things. At each period 

 of moulting, and during every metamorphosis, the old 



