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. Many instances could
be given to prove that progression and retrogression are the two main
principles of evolution at large. Hence the conclusion, that our
analysis must dissect the complicated phenomena of evolution so far as
to show the separate functions of these two contrasting principles.
Hundreds of steps were needed to evolve the family of the orchids, but
the experimenter must take the single steps for the object of his
inquiry. He finds that some are progressive and others retrogressive and
so his investigation falls under two heads, the origin of progressive
characters, and the subsequent loss of the same. Progressive steps are
the marks of elementary species, while retrograde varieties are
distinguished by apparent losses. They have equal claim to our interest
and our study.

As already stated I propose to deal first with the elementary species
and afterwards with the retrograde varieties. I shall try to depict them
to you in the first place as they are seen in [16] nature and in
culture, leaving the question of their origin to a subsequent
experimental treatment.