METAMORPHOSIS GENERALLY. 53 



tioned, is apparent in the amphibious reptiles, which 

 are the lowest of the vertebrate class. Incomplete 

 metamorphosis, again, is shown under various modifica- 

 tions. In some, it is confined to a simple increase in 

 the number of the feet, as in the apterous Myriapoda, or 

 Centipedes ; in others, the whole appearance of the ani- 

 mal is changed, as in the frogs. In all these cases, 

 however, the changes appear to be but two in number ; 

 namely, the perfect form being assumed immediately 

 after that is lost, in which the animal emerged to life. 

 The tadpole changes to the frog, Avithout any inter- 

 mediate stage of existence, or any intervening change of 

 form : this is, therefore, the lowest, or most incomplete, 

 sort of transformation ; since the first, or larva state, 

 merges immediately into the perfect creature, without 

 the intermediate change of the pupa, or chrysalis. The 

 extraordinary facts regarding the metamorphosis of the 

 crabs, disclosed by Mr. Thompson, show that these ani- 

 mals, also, never undergo the pupa transformation, 

 although their change is much more remarkable for the 

 complete alteration effected in their external appearance, 

 (49.) So soon, however, as we approach the winged 

 insects, — among which, as being the typical perfection of 

 annulose animals, we should expect to find metamorpho- 

 sis in its highest developement, — then we see a third 

 stage of existence added to the other two. The larva, 

 before it becomes a perfect insect, changes into a pupa, 

 or chrysalis ; and we have thus a threefold transform- 

 ation. Here, again, we find those minor variations by 

 which that gradual progression which Nature delights 

 in can be traced. At first, as among the grasshoppers, 

 the larva and pupa are only to be known from the per- 

 fect insect by their want of wings ; next, the pupa be- 

 comes inactive, but still retains something of the external 

 form of the state it had previously lived in. It is here, 

 then, that we may draw our imaginary line of demarc- 

 ation between an incomplete and a complete transform- 

 ation. When the several stages of an insect's life are 

 marked by such a striking difference in their form, that 



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