1898 | THE ORIGIN OF GYMNOSPERMS 163 
but one tendency of a much more extensive group, for which the 
name Cordaites may be extended for convenience. Cordaites in 
this larger sense occur in such association with groups of homo- 
sporous eusporangiate Filicales, and approach them so much 
nearer in the important morphological structures mentioned than 
they do living heterosporous Filicales, that an independent 
heterosporous line is suggested. If such be the case, in the 
passage from the Marattia forms to the Cordaites form both 
heterospory and the retention of the megaspore were attained, 
and probably siphonogamy begun. 
THE SEED HABIT. 
The evolution of heterospory seems simple enough. The 
physiological differentiation of the spores was complete when 
prothallia became persistently dicecious. This division of labor 
is to be expected in the case of two such distinct functions as 
the production of antheridia and archegonia. A prothallium 
producing both sex organs equally well may be regarded as in 
a state of equilibrium, an equilibrium which is disturbed by any 
conditions which favor the production of one sex organ rather 
than the other, in this case probably nutritive conditions. This 
disturbance of the equilibrium of a bisexual prothallium would 
certainly find an expression first in a dicecious tendency, and 
finally in a dicecious habit. With the habit once fixed the mor- 
phological differentiation of spores becomes inevitable, since the 
nutritive requirements of the two prothallia are so different. 
The evolution of heterospory seems to be one of the simplest 
of selective processes, with inequalities of nutrition to furnish 
the variations. From this point of view it would seem natural 
to expect that it may have been derived frequently from homo- 
Spory. 
The retention of the megaspore, however, does not seem tobe 
So simple a problem. Ina certain sense it is correlated with the 
reduction of the gametophyte, since retention would not seem 
practicable until reduction had proceeded far enough to make 
the gametophyte endosporic. Even greater reduction, however, 
