THROUGH CUMULATIVE SEGREGATION. 
271 
mutual sterility), unless observation shows that they are of 
common descent. These two considerations sufficiently explain 
why the varieties of one species are so seldom reported as 
mutually infertile. Notwithstanding this, the experiments of 
Gartner and of Darwin, already referred to at length, seem to 
show that Segregate .Fecundity and Vigour may arise between 
varieties that spring from one stock. In view of these cases, we 
must believe that in the formation of some, if not many, species, 
the decisive event with which permanent divergence of allied 
forms commences is the intervention of Segregate Fecundity or 
Vigour between these forms. Positive Segregation, in the form 
of Local, Germinal, or Floral Segregation producing only tran- 
sitory divergences, always exists between the portions of a species 
that has many members, but as it does not directly produce the 
Negative Segregation which is, in such cases, the necessary ante- 
cedent of permanent divergence, we cannot, in accordance with 
the usage of language, call it the cause of the permanent diver- 
gence. Moreover, though it may be in accordance with ordinary 
language to call the Negative Segregation, which is the immediate 
antecedent of the permanent divergence, the cause of the same, it 
will be more correct to call the coincidence of the Negative and 
Positive Segregations the cause, and still more accurate to say that 
the whole range of vital activities (when subjected to the limita- 
tions of any sexual incompatibility that corresponds in the groups 
it separates to some previous but ineffectual Local, Germinal, 
or Floral Segregation), will produce permanent divergence. 
In many cases not only is the entrance of Impregnational 
Segregation the cause of the commencement of permanent diver- 
gence, but its continuance is the cause of the continuance of the 
divergence. The clearest illustration of this is found in the case 
of plants that are fertilized by pollen that is distributed by the 
wind. All the higher, as well as the lower, groups of such plants 
would rapidly coalesce if each grain of pollen was capable of 
producing fertilization, with equal certainty, promptness, and 
efficiency, on whatever stigma it might fall. We may also be 
sure that, with organisms that depend upon water for the dis- 
tribution of their fertilizing elements, Impregnational Segrega- 
tion is an essential factor in the development of higher as well 
as of lower taxonomic groups. 
It is important to observe that, in the cases under considera- 
tion, the inferior fertility or vigour resulting from the crossing of 
