PRINCIPLES OP METAMORPHOSIS. 167 



germes in the newly -born caterpillar) and Dr. Herold 

 (asserting that these skins are successively produced from 

 the rete mucosum), but simply whether we are to consider 

 with Kirby and Spence that these germes are pre-existent, 

 though not perceivable even by the application of the most 

 powerful microscope, but which subsequently, " by the con- 

 stant accretion of new matter in a proper state, are gradually 

 developed ;" or with Dr. Herold, that they do not pre- 

 exist as germes, but are successively formed from the rete 

 mucosum. Having, however, divested the arguments of the 

 latter from the charge of atheism which has been attempted 

 to be forced upon them, I cannot consider that there are no 

 grounds for the adoption of the latter in preference to the 

 former. It is true that Dr. Herold discovered the organs of 

 sex in the newly-excluded larva ; but these and other organs 

 of the perfect butterfly are those which, from their very 

 nature, must be regarded as requiring the greatest and most 

 gradual degrees of developement ; but the case is quite dif- 

 ferent with such temporary organs as the skins of the cater- 

 pillar. But let us look to facts : Messrs. Kirby and Spence 

 are unable to adduce any in support of the existence of in- 

 visible germes, but, according to Dr. Herold, there is not the 

 least trace in the young larva of the new skin, but this first 

 originates towards the end of the first period of the cater- 

 pillar's life, a few days only before the old one is stripped off. 

 It is then observed that the mucous and muscular layers of 

 the skin separate all round from the epidermis, and then 

 clothe themselves upon the superior surface with a new 

 epidermis. The developement of this new external skin 

 occupies two or three days, during which the caterpillar 

 appears sickly, and takes but little nourishment : the old 

 skin then splits longitudinally along the back, and the cater- 

 pillar emancipates itself from its old envelope by drawing 



