1915] Quantitative Entomology ave 
purposes, since they emphasize the dominant groups and their 
most evident characteristics. Keys can be wisely restricted 
to single characters wherever possible and the use of ‘“‘usually”’ 
or ‘‘rarely’’ in connection with a character should be rigorously 
excluded. Key characters should be so selected that the 
differentiation is most certain and evident. All differentiating 
characters must be definite degrees of variation. 
The diagnosis of a species should be limited to differentiating 
characters. A great deal that finds its way into the description 
of a new species is descriptive and not diagnostic and is worse 
than useless for the purpose of identification. On the other 
hand, very few descriptions are exhaustive enough. Every 
character that rightfully belongs in the diagnosis of one species 
of .a genus should appear in the diagnosis of every other species 
of that genus and each genus will have its own set of differ- 
entiating characters. 
The contention is, first, that there exists in each genus a 
set of definite quantitatively measurable variants which con- 
stitute the diagnostic resources for that genus and that all of 
them should be included in each specific description, but that 
a distinction should be made between them for use in keys, 
only those most evident or tangible being employed and that 
the key should be arranged solely for convenience in 
identification. 
DESCRIPTIVE ENTOMOLOGY. 
What we know as the specific descriptions of insects even 
in their most elaborate form are too meagre to be looked upon 
as fairly representing what descriptive entomology should 
accomplish. 
For a few insects the accounts of the structure that have been 
published are perhaps sufficiently voluminous, but only the 
beginning has been made in the approach to the ideal which 
I wish to urge as the goal for future work. 
Descriptive Entomology is concerned with, Ist, a study of 
size and form; 2nd, surface differentiation; 3rd, color and 
pattern and 4th, internal structure. 
Size and Form should be so studied as to become expressible 
in terms of the dynamics of growth. We must discover and 
measure the intensity of the determining factors. A few 
