XXV1 INTRODUCTION. 
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 vertebree, &e. 
3. Collateral metamorphoses, comprising those permu- 
tations of form and function manifested m 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 
naturalists 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 
have an illustration of the degree of change possible in 
ageregates of organisms under existing circumstances. 
