434 THE STRUCTURE OF EVOLUTIONARY THEORY
thought (dating to Lamarck's distinction of progressive increments from lateral
branches—see Chapter 3), de Vries subdivided, by their presumed phylogenetic
effect, these taxa of similar genetic status. He labelled mutations yielding phyletic
novelties as "elementary species," while phenotypic departures still linked to
parental morphologies became "varieties." De Vries then made a further
subdivision among varieties, distinguishing taxa formed by loss of a character
("retrograde varieties") from those that may seem more advanced than the parent
but really display nothing new (atavistic reappearance of characters present in
closely related species, for example, or simple enhancement of a character already
present). De Vries wrote (1905, pp. 246-247):
There is a real difference between elementary species and varieties. The
first are of equal rank, and together constitute the collective or systematic
species. The latter are usually derived from real and still existing types.
Elementary species are in a sense independent of each other; while varieties
are of a derivative nature... We have assumed that the first came into
existence by the production of something new, by the acquirement of a
character hitherto unnoticed in the line of their ancestors. On the contrary,
varieties, in most cases, evidently owe their origin to the loss of an already
existing character, or in other less frequent cases, to the reassumption of a
quality formerly lost. Some may originate in a negative way, others in a
positive manner, but in both cases nothing really new is acquired.In his most forthright statement about the differing phyletic roles of
progressive and regressive mutations, de Vries then stated (1905, p. 15):
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.The logic of de Vries' system may be sound, but he faced—as he well
understood—a major empirical dilemma. He had found consistent mutations in
only one lineage, the genus Oenothera, and with high frequency only in the species
O. Lamarckiana. (He noted an isolated example or two in other lineages,
particularly in a plant with the intriguing name of "peloric toad-flax," but found no
consistently mutable form besides Oenothera. He also tested other species of
Oenothera, but found most immutable or, in the case of O. biennis for example,
much less subject to alteration than O. Lamarckiana. In