440 ON THE TRtBES 



no one can doubt the fact of both of these being cases of 

 complete ecdysis, only differing from that of Lepidoptera 

 because in the former animals the internal envelope is al- 

 ways of the same form as that of which it is to take the place. 

 The truth perhaps is, that we ought only to allow two states 

 to every animal, a perfect and an imperfect state. Then, 

 by the reflection that no animal out of the circle of Acrita 

 can ever arrive at its perfect state except by means of Me- 

 tamorphosis, and that when perfect it can never again be 

 subject to this change of form, though it may still moult 

 or shed its external envelope, we may be able, if not to com- 

 prehend the cause, at least to know the effect of some of 

 the most puzzling phgenomena in nature. The true crite- 

 rion of animal as well as vegetable perfection is the ability 

 to continue the species ; hence some of the Vertebrata, as 

 well as Amndosa, gaining this faculty before they have ar- 

 rived at their proper type of form, metamorphosis ceases, 

 and they preserve the shape of larvse. 



But if a complete ecdysis may sometimes create a total 

 change in the external appearance of the animal, the fact, 

 however astonishing, is nothing in comparison with the in- 

 ternal metamorphosis which accompanies it, and of which 

 as yet no philosopher has been able to give any satisfacto- 

 ry explanation. The generalization indeed by which we 

 have reduced the moulting of a bird's feathers and the 

 metamorphosis of a butterfly to one principle, may appear 

 to be strained beyond its proper limits ; yet if we contem- 

 plate the regular gradation from one to the other, how 

 truly for instance the inactive pupa of a Beetle corresponds 

 with the agile nympha of a Gryllus, how this ecdysis in an 

 Apterous Gryllus corresponds with the sloughing of a spider; 

 and this again with the annual renovation of the serpent. 



