Such
varieties are, strictly speaking, not to be treated in the same way as
elementary species, though they often are. We shall designate them by
the term of "retrograde varieties," which clearly indicates the nature
of their relationship to the species from which they are assumed to have
sprung. In order to lay more stress on the contrast between elementary
species and retrograde varieties, it should be stated at once, that the
first are considered to have originated from their parent-form in a
progressive way. They have succeeded in attaining something quite new
for themselves, while retrograde varieties have only thrown off some
peculiarity, previously acquired by their ancestors.
The whole vegetable kingdom exhibits a constant struggle between
progression and retrogression. Of course, the great lines of the general
pedigree are due to progression, many single steps in this direction
leading together to the great superiority of the flowering plants over
their cryptogamous ancestors. But progression is nearly always
accompanied by retrogression in the principal lines of evolution, [15]
as well as in the collateral branches of the genealogical tree.
Sometimes it prevails, and the monocotyledons are obviously a reduced
branch of the primitive dicotyledons. In orchids and aroids, in grasses
and sedges, reduction plays a most important part, leaving its traces on
the flowers as well as on the embryo of the seed.
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