1922] BUCHHOLZ—VASCULAR PLANTS 277 
does not belong rigidly to the categories of developmental selection, 
for it is subject very largely to external environmental conditions. 
It is not entirely in the external environment, however, since 
internal physiological correlations are concerned. It is clearly a 
type of selection standing between natural selection and de- 
velopmental selection, as previously indicat 
Summary 
Embryonic selection and gametic selection represent the 
important forms of developmental selection found among pterido- 
phytes. Among gymnosperms the gametophytes plus the embryo 
usually take part in this competition, and among angiosperms it 
is typically a competition between male gametophytes represented 
by the pollen tubes. The important reproductive developmental 
process is merely shifted to an earlier stage of the life cycle in passing 
from ferns to angiosperms, where vegetative and other less important 
forms of developmental selection are also found. 
Developmental selection in relation to natural selection 
and mutation 
It is very evident that a good selective process, whether develop- 
mental, artificial, or natural selection is meant, should meet at least 
the following four requirements. It should (x) start the competi- 
tion simultaneously, (2) take place under uniform conditions, (3) 
measure comparable merit, and (4) rigidly eliminate the great 
majority that fall below the standard. 
First it should launch the individuals who participate into the 
competition with an even start. This is a primary requirement in 
any kind of a performance race. Well fitted is the process of 
fertilization to launch the competition of embryos, as this is a 
relatively sudden event, one which may happen only occasionally, 
and may be simultaneous in producing several or a large number 
of zygotes. Similarly the shedding of sperms from an antheridium 
provides an even start in the competition of sperms, with a definite 
goal to be reached through their activities. Pollination, especially 
when the pollen is transferred in masses or clumps by insects, is 
another more or less sudden event, which launches the competition 
