412 Dawson—Bearing of Devonian Botany on 
without any external cause. : 
(6.) With regard to the introduction of specific types we have 
t infe i 
ties, may result from derivation, this by no means excludes the 
idea of primitive specific types originating in some other way. 
to discover, if possible, what are elementary or original types, 
and, having found these, to enquire as to the law of their crea- 
tion, 
(7.) In prosecuting such questions geographical relations 
must be carefully considered. When the floras of two succes: 
sive periods have existed in the same region, and under cireum- 
stances that render it Eitan that plants have continued to 
J 
broken continuity of fi 
I ire, however, under 
fone 
‘orth Am 
