1915} On Proper Generic Concepts 87 
including evolutional trend or anatomical convergence and 
parallelism, valuation of characters. 
In other words, in attempting to fix the proper generic con- 
cept in a given group, we must consider the position of that 
group in the phylogenetic scale (orientation); the response of 
that group to the factors of the environment (adaptation) ; 
and the comparative importance of the various internal and 
external anatomical structures of all instars for indicating 
relationships and lines of development both within and without 
that group (inheritance and valuation). The detailed con- 
sideration of these principles leads us to the following reasons 
for restricted genera in most of the muscoid groups. 
(1). Smaller generic value in young stocks owing to greater 
approximation of the generic stems and their branches, carrying 
with it the necessity for generic recognition of such stems or 
types of development from a direct but rather remote common 
origin, in order to avoid indefinite diagnoses and secure concise 
generic concepts. Here the principle of orientation figures. 
The majority of muscoid stocks stand near the limits of present 
arthropod specialization. They represent the numerous twigs 
of the phylogenetic tree. As such they are, on the whole, 
young stocks yet in process of evolution, still approaching their 
maturity, full of transitionals. These stocks exhibit, each 
within itself, certain definite lines of development issuing 
directly from a common source and marking generic stems, 
which are often connected by transitional species. Groups of 
generic stems that happen to be connected throughout by 
transitional species can not be treated as a single genus, on 
account of their diverse combinations of characters. Such 
stems must be separately recognized. They can be satisfac- 
torily defined as genera by employing atavic and subatavic 
characters, or those afforded by structures not especially sub- 
ject to adaptation. Here the principle of adaptation enters. 
We have now to provide for the taxonomic disposal of the tran- 
sitional species. They may be defined as lettered but un- 
named subgenera of those restricted genera in common with 
which they show the most characters of phylogenetic 1m- 
portance. Older stocks do not exhibit such conditions as ob- 
tain here. Their generic stems are farther apart, the branches 
therefrom less numerous, and many of their transitionals have 
disappeared. (See diagram). 
