1922] BUCHHOLZ—VASCULAR PLANTS 269 
5. Gametophytes are so reduced in size that only one arche- 
gonium is produced, making selection between embryos of separate 
fertilization impossible, as, for example, Marsilia and Pilularia. 
Selection between gametes 
Among all of these pteridophytes another form of developmental 
selection may be recognized. Doubtless a selection occurs among 
the male gametes as they swim to the archegonia. That the 
archegonia attract the sperms chemotropically has long been 
known. The gametic selection is therefore a measure of their 
response to this stimulus. While it may be largely a matter of 
chance which of the many sperms that reach the archegonium and 
swarm about its neck actually reach the egg to effect the fertiliza- 
tion, there can be no doubt that the less active sperms or those 
otherwise defective would be eliminated in the race to reach the 
egg. If only the most vigorous sperms take part in fertilization, 
and there seems to be very good ground for this, certainly a form 
of gametic selection is to be recognized. It may be noted in passing 
that with its return to aquatic life, the natural sphere of swimming 
sperms, Marsilia has exchanged one form of developmental selection 
for another. Embryonic selection was made impossible and lost 
through reduction of archegonia, but gametic selection was doubt- 
less facilitated when this fern returned to the aquatic habit. 
Gametic selection is not a new suggestion, having been suggested 
by THomson (36) for animals, and it is probably in part along this 
line that the principle of developmental selection may be found to 
apply somewhat generally to the animal kingdom. 
It must be remembered that no reduction divisions occur in 
the formation of sperms in ferns, nor is any special form of cell 
division known or recognized here which might bring about genetic 
changes. In animals the formation of sperms is accompanied by 
a chromatin reduction both equational and differential, a condition 
shared by some algae, notably Fucus. 
OTHER FORMS OF DEVELOPMENTAL SELECTION.—There are still 
other forms of developmental selection which must be taken into 
account. One of these is illustrated by the selection which takes 
Place at a certain stage of development in Selaginella between the 
