Limits of Species in the Diatomacese. By Thomas Comber. 457 
with pollen from another flower on the same plant, they are some- 
times (though rarely) a little more fertile ; if fertilised from another 
individual or variety of the same species, they are fully fertile ; hut if 
with pollen from a distinct species, they are sterile in all possible de- 
grees, until utter sterility is reached. We thus have a long series, 
with absolute sterility at the two ends ; at one end due to the sexual 
elements not having been sufficiently differentiated, and at the other 
end to their having been differentiated in too great a degree, or in 
some peculiar manner.” 
Regarding this as a so-called “ law of Nature,” must not a natural 
division into “ species ” result from its operation ? 
The cause of the cross-sterility, after a certain differentiation of 
the sexual elements has taken place, will probably some day be dis- 
covered, and will then be found to be an entirely natural one. This 
does not in any way affect the argument. We have to do, not with 
its cause, but with its effect ; and that is to cut up the originally 
continuous chain of related forms into separate sections, incapable of 
completely fertile crossing. Such sections are what appear to me 
to be natural species, sufficiently distinct and separable from each 
other, without any reference to special creation. Hybrids between 
them can be raised and maintained artificially, or may even occur 
in a state of nature ; but in the latter case they do not last. In 
the struggle for existence they cannot contend against their more 
perfectly fertile competitors, and are suppressed. 
So long as the individuals and varieties forming a group are 
capable of interbreeding freely, their offspring, being fully fertile, 
survive in the struggle for existence. The characters of the entire 
group may be gradually changed by the action of natural selection, 
but through continual crossing they are changed together. Such a 
group constitutes only a single species. By the interbreeding of all, 
even the extreme, forms of it, a series of intermediate forms is pro- 
duced, however unlike the extremes may be. A visit to a poultry 
or dog show will give an example of how far they may differ. When, 
however, through isolation, geographical or otherwise, and from part 
of the group being exposed to different conditions of life, a differen- 
tiation in the sexual elements of the individuals composing that 
part is brought about, so that they can no longer produce fully fer- 
tile offspring when crossed with the others, the more or less sterile 
hybrids no longer hold their own. They are handicapped out of 
existence; by their elimination a gap in the series of intermediate 
forms is caused ; a new “ species,” as I understand the term, comes 
into existence ; and it is afterwards maintained, distinct from the 
original species, by the same sterility, in a greater or less degree, 
of the hybrids. 
Mr. Darwin, with his characteristic accuracy and thoroughness, 
after describing the more or less complete sterility of what he terms 
the “ illegitimate ” unions of heterostyled plants, points out that “ we 
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