308 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
the same seed than by pollen produced from any other one seed, then 
germinal segregation is the result. 
In order to secure this kind of segregation it is not necessary that 
the flowers fertilized by pollen from the same plant should be more 
fertile, or the seeds capable of producing more vigorous plants than the 
flowers fertilized by pollen from another plant. All that is required is 
that of the seeds produced a larger number shall be fertilized by the 
pollen of the same plant than by the pollen of any other one plant. 
This form of segregation 1s closely related to local segregation on one 
side, and to social segregation on the other. It however differs from 
the former in that it does not depend on migration or transportation, 
and from the latter in that it does not depend on social instincts. 
13. Floral segregation is segregation arising from the closest form of 
self-fertilization, namely, the fertilization of the ovules of a flower by 
pollen from the same flower. 
Many plants that in their native haunts are frequently crossed by 
the visits of insects depend entirely on self-fertilization when trans- 
ported to other countries where no insect is found to perform the same 
service for them. The common pea (Piswm sativum) is an example of 
a species that habitually fertilizes itself in England, though Darwin 
found that it was very rarely visited by insects that were capable of 
varrying the pollen.* Darwin also mentions Ophrys apifera as an 
orchid which ‘has almost certainly been propagated in a state of 
nature for thousands of generations without having been once inter- 
crossed.”t 
A fact of great importance in its bearing on the origin of varieties 
should be here noted. Any variation, arising as a so-called sport, in 
any group of plants where either of these principals is acting strongly, 
will be restrained from crossing, and will be preserved except in so far 
as reversion takes place. Now there is always a possibility that some 
of the segregating branches of descent will not revert, and that 
through the special character which they possess in common, they will 
some time secure the services of some insect that will give them the 
benefit of cross-fertilization with each other without crossing with 
other varieties. The power of attaining new adaptations may be 
favored by self-fertilization, occasionally interrupted by inter-breeding 
with individuals of another stock; for the latter is favorable as intro- 
ducing vigor and variation, and the former as giving opportunity for 
the accumulation of variations. 
(b) Impregnational segregation. 
Impregnational segregation is due to the different relations in which 
the members of a species stand to each other in regard to the possi- 
bility of their producing fertile offspring when they consort together. 
* See “ Cross and self-fertilization in the vegetable kingdom,” p. 161. 
t See ibidem, p. 439. 
