XXVI INTEODUOTION. 



its passage from the embryonic to the adult condition, 

 or from the simple and incomplete to the complex and 

 perfect. 2. Ascending metamorphoses, including those 

 changes of form manifested in the same adult organism 

 by the several parts of which it consists those parts 

 being typically identical or homologous, such as the 

 parts of the flower, or, in animals, the vertebrae, &c. 

 3. Collateral metamorphoses, comprising those permu- 

 tations of form and function manifested in homo- 

 logous organs in the different groups of organisms, 

 classes, orders, genera, species, &c. 



Thus, in the first instance, we have a comparative 

 examination of the form of each or any separate part 

 of the same individual at different epochs in its life- 

 history ; in the second we have a similar comparison 

 instituted between the several parts of the same organ- 

 ism which originally were identical in appearance, but 

 which have in course of evolution altered in character. 

 In the third form we have the comparative view not 

 of one organ at different times, nor of the several parts 

 of one organism, but of the constituent elements per- 

 taining to those aggregates of individuals to which 

 naturahsts apply the terms classes, orders, &c. 



In successive metamorphosis we have a measure of 

 the amount of change and of the perfection of struc- 

 ture to which each separate organ attains. 



In ascending metamorphosis we have a gauge of 

 the extent of alteration that may take place in the 

 several homologous organs under existing circum- 

 stances. 



In collateral metamorphosis, in the same way, we 

 liave an illustration of the degree of change possible in 

 aggregates of organisms under existing circumstances. 



