understand it, from an interpretation of the evidence now 
available, as well as to point out the gaps where intensive 
research is required for further clarification. It is my hope 
that this effort may stimulate the students of evolution 
to turn some of their attention to this intricate and per- 
plexing group of plants. 
The problem of the origin and phylogeny of various 
plant groups and families has occupied the minds and in- 
terest of a number of outstanding botanists from Lin- 
naeus to our time. All have recognized that in nature a 
system prevails within which everything seems to follow 
a pattern of progressive differentiation from simplicity to 
complexity. While Linnaeus’ sexual system can hardly 
be associated with progressive differentiation, the sys- 
tems of Bentham and Hooker or of Engler and Prantl, 
to mention but two, in essence convey this idea. How- 
ever, progressive differentiation in the light of our mod- 
ern species-concept assumes an entirely different role and 
meaning from that which was understood and employed 
by the earlier workers. A glance at any of the proposed 
‘‘natural’’ systems is sufficient to make us recognize that 
they are based on a continuous modification along a 
straight line of descent. We are informed today, how- 
ever, that species could also have been derived simul- 
taneously or consecutively from a common ancestor or 
from the fusion of one or more common ancestors. Pro- 
gressive differentiation in the light of these latter princi- 
ples assumes an entirely new and far reaching significance. 
If we wish to understand the origin or phylogeny of 
any group, regardless of its taxonomic status (genus, 
subfamily, family, ete.), we must first understand the 
basic unit: THe Species. A lengthy discussion about 
the nature of species does not, however, lie within the 
scope of this paper, and I must assume that the reader 
is fully acquainted with the subject. It is sufficient, there- 
[ 58 | 
