250 BOTANICAL GAZETTE [APRIL 
entirely incapable of experimental proof. Likewise Roux’s theory 
of intra-selection or the battle of the parts, a supposed struggle 
between the various organs of the body of a developing individual, 
is not a struggle between different individuals, but only between 
differentiating tissues and organs. Developmental selection, on 
the other hand, occurs between definite individuals which may 
be unicellular or multicellular, but the process is not intracellular. 
This can easily be demonstrated and is capable of being subjected 
to observation and experimental study. 
The several isolation theories, as well as the other theories of 
species forming auxiliary to natural selection, have neglected any 
significant allusion to the type of illustrative material discussed in 
this paper. Developmental selection is different also from the 
theories of sexual isolation, physiological selection, or mechanical 
selection. 
A definite réle is not to be denied for natural selection, but it is 
not the only selective process. In developmental selection, we 
have a supplementary form of selection which occurs at other 
times during the life cycle, one which meets some of the most 
serious objections which have been raised against natural selection. 
It is capable of playing on mutations as well as other forms of varia- 
tion, bringing about definite results in evolution. In fact, a real 
species forming réle may be claimed for developmental selection, 
if we grant that such a réle is to be found in any selective process. 
During the ontogeny of higher plants, therefore, there are a number 
of stages in the life cycle when a definite competitive selection 
between individuals occurs. In addition to natural selection, 
there is also this definite struggle between supernumerary gameto- 
phytes, when these are dependent on sporophytes, between super- 
numerary embryos as found in the polyembryony of gymnosperms 
and most ferns, or between excessive numbers of gametes. 
In plants, natural selection, as it is ordinarily understood, 
occurs in the environment, when seeds or spores germinate in oF 
on the soil, or when vegetative organs, such as roots, rhizomes, 
stolons, etc., from several neighboring plants of similar or different 
species give rise to new individuals in a crowded stand and in close 
competition. For animals, natural selection is usually understood 
to begin at birth, or when the young first come into contact wit 
