PRINCIPLES OF METAMORPHOSIS. 165 



microscopic observer, who, from a most elaborate indeed, 

 it may be affirmed, the most elaborate investigation of its 

 kind hitherto made upon the cabbage butterfly in its dif- 

 ferent states, has arrived at the contrary conclusion namely, 

 that the successive skins of the caterpillar, the pupa case, &c., 

 do not pre-exist as germes, but are formed successively from 

 the rete mucosum, which itself is formed anew upon every 

 change of skin ; these formations being attributable to the 

 action of a power which he terms the vis formatrix. 



Against the latter theory, Messrs. Kirby and Spence have 

 raised various objections, none of which appear to me 

 to be of sufficient weight to overthrow it in favour of 

 the old theory of Swammerdam. The first of these objec- 

 tions is, that Dr. Herold employs the high-sounding term, 

 ris formatrix, to conceal his own ignorance of causes, when 

 endeavouring to penetrate within the sanctum sanctorum, 

 which is not permitted to vain man to enter a plan 

 adopted in all ages, and instanced in the adoption of such 

 words as Plastic Nature, Epigenesis, Panspermia, Idea semi- 

 nalis, Nisus formativus, &c. It is evident from these ob- 

 servations, that these authors would endeavour to fix upon 

 Dr. Herold a charge of infidelity, in supposing that the 

 body of an insect possessed of itself a power to mould 

 itself according to its own formative will and power; and 

 this is evident, because in a subsequent page they adduce 

 the fact, with which every physiologist is acquainted, that 

 the developements and acquisition of new parts and organs, 

 by insects undergoing metamorphoses, have taken place ac- 

 cording to a law which regulates their numbers, kinds, and 

 times, since the first creation : but surely they are not 

 authorized by Dr. Herold's views and statements, in wielding 

 against him this powerful (if maintainable) argument, since 

 it must be evident, that by the term vis formatrix, Dr. 



