20 THE SCOLYTID BEETLES. 
each degree south, while at the same latitude there will be a difference 
of about four days for each 400 feet difference in altitude — four days 
later for each 400 feet higher, and four days earlier for each 400 feet 
lower. Thus a difference of 7| degrees of latitude at the same eleva- 
tion would mean a difference of about thirty davs in the beginning 
of activity or any other event, depending on a given average tem- 
perature, while a difference of 3,000 feet in altitude at the same 
degree of latitude would cause an equal difference in these phenolog- 
ical events. 
In the fall of the year the beginning of hibernation and other events 
will be earlier northward and later southward at localities of the 
same elevation, or earlier at higher altitudes and later at lower alti- 
tudes in the same latitude. 
Of course there are exceptions to these rules, especially in regions 
like that of California, where remarkably abnormal conditions as to 
influence of altitude and latitude prevail, as also in the case of southern 
and northern exposures, sandy dry soils, and wet clayey soils or bogs. 
In such cases the estimates must be corrected so as to allow for three 
or four days later for the beginning of activity, etc.. under average 
colder conditions, or three or four days earlier for average warmer 
conditions. The best indication of the rate of difference between two 
localities is found in the average difference in the dates of opening 
of the buds or flowers of some indigenous species of forest trees com- 
mon to both localities, and especially of a species of conifer subject 
to the attack of a given Dendroctonus beetle. 
A knowledge of the facts relating to this principle is of especial 
importance as a basis for recommending or executing beetle-control 
policies, since success depends largely on a Jcnoxdedge of the proper 
time to begin and end certain timber-cutting or barling operations for 
tin destruction of the broods of the beetles. 
When, as is usually the case, the seasonal-history data have been 
collected at different latitudes and altitudes within the range of the 
species, the discussion under each species is based on a probable 
average. But when the data have been collected in one locality the 
discussion relates to that locality, and the probable differences are esti- 
mated for other localities. While there is yet much to be determined 
in regard to the rate of difference between different localities at the 
same latitude or elevation in the same region and the influence which 
different latitudes and altitudes exert (in different species, sufficient 
evidence is at hand regarding some of the species of this genus and the 
regions occupied by them to warrant certain preliminary conclusions 
a- a basis for action and further study. 
