82 BOTANICAL GAZETTE ' - [aveust | 
and yet it is clear that heterospory arose several times, and probably — 
many times, independently as the natural result of a general tendency — 
among pteridophytes. To put into the same genetic group all 
heterosporous pteridophytés would be regarded now as a morpho- . 
logical absurdity; and yet there have been repeated suggestions of 
relationship, especially among conifers, on the basis of certain features — 
of the female gametophyte, for example, features which represent a 7 
Stage in a general change that may occur in a number of independent 3 
lines. a 
In passing from the ancient to the modern gymnosperms, it becomes — 
evident that groups differ as to the rapidity with which they respond — 
to a general tendency to change, and it, is this difference that helps 
to constitute groups. A modern group, for example, may associate a : 
number of ancient features with others that are recent; or all of the — 
ancient features may have been changed. This has been called the 
“lagging behind” of certain structures, but it should not imply that — 
they are held back and will comé forward later ; it simply means that — 
for some reason they have not responded to the general tendency 4 
among gymnosperms to change ina certain direction. The reten- 
tion of an old structure must not be confused with the reappearance 4 
ofan old structure. For example, it seems clear that the most ancient 
gymnosperms were large-leaved forms, from which the small-leaved 
conifers were derived; and yet small-leaved pteridophytes were — 
probably more ancient than large-leaved ones. If this be true, the 
appearance of small leaves among conifers is the reappearance of 
an ancient feature, and not its retention. To prove the retention — 
of an ancient feature demands the establishment of its phylogenetic 
continuity. E 
PHYLOGENY | d 
Before sketching the general evolutionary tendencies among 
gymnosperms, it will be of service to outline what seems to be i 
from citations. Those who know the facts and the investigators dog 
not need their recital; and those who do not know them would only . 
be confused by their recital. 4 
