TRENDS IN SYSTEMATIC BOTANY
49
siderations are as much the concern of the evolutionist as of the taxonomist; they become the concern of the taxonomist because he is striving
to produce a natural system of classification.
The present review-essay on the trends in systematic botany considers
only the vascular plants. Some of these trends apply equally well to
nonvascular plants, as demonstrated by an upsurge of cytotaxonomic
studies in the bryophytes, and the use of new techniques of importance
to the systematist in such groups as the bacteria, all of which will contribute to an improved classification of the plant resources of the world.
Their further elucidation falls beyond the scope of this article.
Taxonomists themselves are not entirely agreed as to whether the
primary objective of taxonomy is to arrive at a system of classification
as nearly phylogenetic as possible, or one based upon a maximum correlation of attributes (Gilmour, 1951; Heslop-Harrison, 1953, 1954). The
connections between taxonomy and phylogeny have been thoroughly
covered by Turrill (1942). All workers would agree, however, that their
job is to classify the end-products of the evolutionary stream as these
have been cast off. The evolutionary process is sufficiently slow so that
it is rare indeed that the taxonomist is able, within his lifetime, to witness the creation of some new unit or the extinction of an old one. The
job consists primarily in placing correctly the groups at hand, which
will ordinarily change imperceptibly during the lifetime of the biologist
(unless man himself has brought about environmental changes), whether
the organism belongs to a rapidly evolving complex or to a most stable,
perhaps quiescent, evolutionary line.
The evolutionary stream may be thought of as breaking up into separate currents or eddies whenever an isolation mechanism comes into
play. Isolation may be caused by ecologic, physiologic, cytologic, or genetic factors. The reader is referred to a concise review of many of these
mechanisms which is provided in the last chapter of the book, "Stages
in the Evolution of Plant Species," by J. Clausen (1951).
Various classifications have been proposed to aid in the discussion of
the evolutionary units, as distinguished from the purely taxonomic or
cytogenetic categories (Gregor, 1939, 1946; Camp and Gilly, 1943;
Valentine, 1949; Gilmour and Heslop-Harrison, 1954), and in the present
paper frequent mention will be made of the now widely familiar terminology of Turesson as given additional precision by Clausen et al. (1939).
The taxonomist uses as building blocks to depict the products of the
evolutionary stream, in ascending order of complexity, the individual,
form, variety and/or subspecies, species, and genus. The geneticist, in
classifying the evolutionary units known to him, likewise starts with the
49
siderations are as much the concern of the evolutionist as of the taxonomist; they become the concern of the taxonomist because he is striving
to produce a natural system of classification.
The present review-essay on the trends in systematic botany considers
only the vascular plants. Some of these trends apply equally well to
nonvascular plants, as demonstrated by an upsurge of cytotaxonomic
studies in the bryophytes, and the use of new techniques of importance
to the systematist in such groups as the bacteria, all of which will contribute to an improved classification of the plant resources of the world.
Their further elucidation falls beyond the scope of this article.
Taxonomists themselves are not entirely agreed as to whether the
primary objective of taxonomy is to arrive at a system of classification
as nearly phylogenetic as possible, or one based upon a maximum correlation of attributes (Gilmour, 1951; Heslop-Harrison, 1953, 1954). The
connections between taxonomy and phylogeny have been thoroughly
covered by Turrill (1942). All workers would agree, however, that their
job is to classify the end-products of the evolutionary stream as these
have been cast off. The evolutionary process is sufficiently slow so that
it is rare indeed that the taxonomist is able, within his lifetime, to witness the creation of some new unit or the extinction of an old one. The
job consists primarily in placing correctly the groups at hand, which
will ordinarily change imperceptibly during the lifetime of the biologist
(unless man himself has brought about environmental changes), whether
the organism belongs to a rapidly evolving complex or to a most stable,
perhaps quiescent, evolutionary line.
The evolutionary stream may be thought of as breaking up into separate currents or eddies whenever an isolation mechanism comes into
play. Isolation may be caused by ecologic, physiologic, cytologic, or genetic factors. The reader is referred to a concise review of many of these
mechanisms which is provided in the last chapter of the book, "Stages
in the Evolution of Plant Species," by J. Clausen (1951).
Various classifications have been proposed to aid in the discussion of
the evolutionary units, as distinguished from the purely taxonomic or
cytogenetic categories (Gregor, 1939, 1946; Camp and Gilly, 1943;
Valentine, 1949; Gilmour and Heslop-Harrison, 1954), and in the present
paper frequent mention will be made of the now widely familiar terminology of Turesson as given additional precision by Clausen et al. (1939).
The taxonomist uses as building blocks to depict the products of the
evolutionary stream, in ascending order of complexity, the individual,
form, variety and/or subspecies, species, and genus. The geneticist, in
classifying the evolutionary units known to him, likewise starts with the