230 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
is that of the atomic theory in the physical sciences, and here it is interesting 
to note that the modern alchemists with their isotopes and electronic 
dissociations are threatening to make chemistry and physics evolutionary 
sciences; at least, they are striving to lay the foundations for a true evolu¬ 
tionary theory of the elements. All schemes of classification aim to be 
phylogenetic. Their rubrics are based without question on evolutionary 
concepts and express the knowledge of our time as to genetic relationships 
between the units classified. 
There is no real question that the species concept as used by Linnaeus, 
so far as it relates to content at any one time, does fit in with the facts of 
evolutionary phylogeny as we know them. Modern research has made us 
more certain than ever that the Linnaean species do by and large constitute 
recognizable groups of more or less freely interbreeding individuals whose 
interwoven pedigrees constitute a specific fabric less diversified than that 
of the genus, family, or order. Only extremists deny the possibility of 
segregating and recognizing such genetic units. Only those who hold that 
all organisms constitute a web of life or vital fabric of descent in which 
each individual pedigree is inextricably woven up with other pedigrees so 
that its identity is lost in that of the whole, or, on the other hand, those 
who hold that the individual pedigrees are the only recognizable and 
differentiable units and that these are all of essentially the same order of 
magnitude, have really cut loose from the use of the species concept. 
The question is today not whether Linnaean species are in general good 
phylogenetic units, but whether, with the discovery and recognition of 
such divisions of these Linnaean species as our small species, or subspecies, 
which also constitute recognizably differentiated groups of interbreeding 
individuals, we should now transfer the term species to these phylogenetic 
units of a lesser order of magnitude, leaving the Linnaean units unrecognized, 
or perhaps—as has sometimes been hinted by the “splitters” might be 
desirable—giving them in many cases generic rank. It is of interest to 
note that such proposals do no violence to evolutionary concepts. Agree¬ 
ment here is practically universal among all ranks of biological students. 
The question turns largely on what is practical. On this point Hall has 
put the evidence very clearly. It is indispensable in our systems of classi¬ 
fication that we recognize and delimit units of both these orders of magni¬ 
tude. The geneticists and the plant geographers can give clear expression 
to their results only by recognizing these lesser units—twigs on the evolu¬ 
tionary tree. It is in them that the variants of genetic modification and 
many times of geographic distribution are displayed. They, or still lesser 
races, varieties, etc., are the real material of evolutionary advance. Their 
recognition in the field and their production in the breeder’s experimental 
plots constitute the two great current means of advance in evolutionary 
.science. On the other hand, the recognition and critical delimitation of 
the Linnaean units is no less important for the geneticist and plant geogra- 
