206 
Douglas Houghton Campbell. 
absence from the early rocks of the green Algag which have no 
calcareous skeleton is due to the extremely perishable tissues of these 
delicate organisms. 
The distribution of the vascular plants has, of course, received 
much attention, but it may be worth while to call attention to 
some of the conclusions that have been drawn from a study of the 
distribution of the existing Pteridophytes and Spermatophytes, and 
their bearing upon the deductions to be drawn from a study of the 
distribution of the Bryophytes. Many of the living genera of both 
Pteridophytes and Gymnosperms occur in a fossil condition, and 
together with their present distribution furnish pretty complete 
data for a study of their geological history. 
The distribution of the Angiosperms of the present day, 
together with what is known of their distribution in the late 
Cretaceous and Tertiary, when many existing genera were already 
differentiated, throws a good deal of light upon the history of the 
group, although it must be remembered that the fossil record is 
much less satisfactory than is the case with the lower vascular 
plants. This is no doubt largely due to the much greater proportion 
of herbaceous species, which were poorly fitted to leave recognizable 
fossil remains. Many types, which there is good reason to suppose 
are old ones, are quite unknown in a fossil condition. 
At the present day there is a nearly uniform sub-polar zone of 
vegetation in which the predominant types, e.g., willows, poplars, 
firs, buttercups, vacciniums, etc., probably represent the descendants, 
but little changed, of the northern glacial flora. Many of these, 
like the Amentaceae and Ranunculaceae, there is good reason to 
suppose are among the more primitive angiospermous types. The 
uniformity in the conditions of this sub-polar zone, as well as its 
practically unbroken extent, readily explain the very slight variation 
in the vegetation of such widely separated countries as Scandi¬ 
navia and Northern Canada. 
Proceeding southward, as might be expected, factors come into 
play that tend towards greater diversity in the flora. The great 
barriers of mountain and ocean, and the greater range of climatic 
conditions due to these and other factors, have had their effect. 
While many of the old tertiary types have survived, undoubtedly 
many new forms have developed, and others have been more or 
less completely destroyed through changes in climate, or from 
causes not so evident. 
The very valuable studies of Asa Gray (see Scientific Papers of 
