xviii
HELEN A. STAFFORD
The diagram (Chart I) summarizes one viewpoint of the phylogenetic relationships between the phyla or divisions of extant organisms. Such a chart, derived mainly from Hyman (5) and Smith (6), is
presented merely as a guide to present day phylogenetic theories, and
should serve only as a useful working hypothesis. Although no two
biologists would ever agree on all details of such a phylogenetic
"shrub," this diagram represents a generalized view held by many
biologists. While comparative biochemistry has already contributed to
our knowledge of phylogenetic relationships, it is hoped that a better
understanding of comparative biochemistry can either strengthen or
weaken the bases on which such a phylogenetic scheme have been
drawn.
The vertical axis is not a time axis, but represents either a crude
measure of the relative morphological and physiological complexity, or
the relative degree of evolutionary advancement of the phyla or divisions named. The time axis would extend downward in a perpendicular
direction below the surface of this paper, and the inclusion of such an
axis would necessitate a three-dimensional diagram. Many of the phyla
or divisions placed here would extend as independently evolving lines
well down in the scale of geological time to the Cambrian period. The
Spermatophyta and the Chordata, on the other hand, arose more recently. Estimates of the time of origin of these major groups are given
in Simpson (3) and Smith (6). The lines interconnecting these major
forms reflect the evidence based on paleontology, and the comparative
morphology, physiology, and biochemistry of modern forms. Groups
that have been separated from each other for a long time, and which
have been evolving independently during this entire period, appear in
the widely diverging lines such as those separating the higher plants
from the higher animals. A more recent separation is that of the Aves
and Mammalia from a reptilian ancestor. In some cases there are known
fossil precursors at the positions just below a main fork, such as the
Psilophytales in the line leading to the radiation of modern vascular
plants (Tracheophyta) and ending in the Spermatophyta. In other
cases, the common precursor below a fork is a hypothetical group
postulated on the basis of comparative studies of modern forms. This is
the case in the primitive acoelomate Bilateria which evolved into the
two major lines, the Protostomia and Deuterostomia.
The "shrub" effect at the base of the diagram indicates a polyphyletic origin from a hypothetical complex of primitive organisms. The
arrangement of the phyla at the base emphasizes the artificiality of the
groups often referred to as Protozoans, Algae, and Fungi, and the difficulty in assigning many of the phyla of microorganisms to either the
HELEN A. STAFFORD
The diagram (Chart I) summarizes one viewpoint of the phylogenetic relationships between the phyla or divisions of extant organisms. Such a chart, derived mainly from Hyman (5) and Smith (6), is
presented merely as a guide to present day phylogenetic theories, and
should serve only as a useful working hypothesis. Although no two
biologists would ever agree on all details of such a phylogenetic
"shrub," this diagram represents a generalized view held by many
biologists. While comparative biochemistry has already contributed to
our knowledge of phylogenetic relationships, it is hoped that a better
understanding of comparative biochemistry can either strengthen or
weaken the bases on which such a phylogenetic scheme have been
drawn.
The vertical axis is not a time axis, but represents either a crude
measure of the relative morphological and physiological complexity, or
the relative degree of evolutionary advancement of the phyla or divisions named. The time axis would extend downward in a perpendicular
direction below the surface of this paper, and the inclusion of such an
axis would necessitate a three-dimensional diagram. Many of the phyla
or divisions placed here would extend as independently evolving lines
well down in the scale of geological time to the Cambrian period. The
Spermatophyta and the Chordata, on the other hand, arose more recently. Estimates of the time of origin of these major groups are given
in Simpson (3) and Smith (6). The lines interconnecting these major
forms reflect the evidence based on paleontology, and the comparative
morphology, physiology, and biochemistry of modern forms. Groups
that have been separated from each other for a long time, and which
have been evolving independently during this entire period, appear in
the widely diverging lines such as those separating the higher plants
from the higher animals. A more recent separation is that of the Aves
and Mammalia from a reptilian ancestor. In some cases there are known
fossil precursors at the positions just below a main fork, such as the
Psilophytales in the line leading to the radiation of modern vascular
plants (Tracheophyta) and ending in the Spermatophyta. In other
cases, the common precursor below a fork is a hypothetical group
postulated on the basis of comparative studies of modern forms. This is
the case in the primitive acoelomate Bilateria which evolved into the
two major lines, the Protostomia and Deuterostomia.
The "shrub" effect at the base of the diagram indicates a polyphyletic origin from a hypothetical complex of primitive organisms. The
arrangement of the phyla at the base emphasizes the artificiality of the
groups often referred to as Protozoans, Algae, and Fungi, and the difficulty in assigning many of the phyla of microorganisms to either the