14 THE AMERICAN. NATURALIST. 
foothold there, could not be maintained against the more aggres- 
sive deciduous flora. That these adverse conditions, physical, 
and biological, have long been active in modifying the character 
of the coniferous flora, and circumscribing its limits, we know 
from the geological records, and that they may be expected to 
continue in the future we have every reason to believe. In 
other words, we have to regard the coniferous flora, taken as 
a whole, as representing a waning type of vegetation, which 
reached its maximum of development in past geologic time, and 
which has since then been slowly but inevitably giving way to 
the deciduous type, which is now in the ascendant. 
The relation between the historical development or evolution 
of the living floras and the facts of their present distribution _— 
will be discussed in a subsequent paper. 
