May, 1923] 
HARPER-SPECIES CONCEPT 
231 
pher in fixing the trends of evolutionary science and of plant and animal 
migration, as well as in the more popular fields of floristics and elementary 
botanical instruction. 
There is little doubt that, if the categories represented by the Linnaean 
species were to be dropped and the term species applied exclusively to the 
smaller units, we should find ourselves resorting to the use of such rubrics 
as species-groups , species-complexes , etc. 
We are passing, in my opinion, at present from a stage of knowledge in 
which the difficulty seemed to be to discover how specific delimitations 
could ever be broken down to a stage of knowledge in which our difficulty 
seems rather to be to find adequate evidence of delimitations between the 
hitherto recognized specific groups. Intensive breeding experiments and 
intensive study of geographic distribution have both contributed to this 
result. Far be it from me to enter the lists for either Mendelism or mutation 
in the great Oenothera controversy. But we must all agree that the mass 
of mutants or segregants, one or both, which have been so painstakingly 
described and pedigreed are well calculated to make the lay botanist, be he 
morphologist or physiologist, more cautious about affixing a species name 
to any chance evening primrose he happens to find by the wayside or in a 
garden. The recognition of the so-called species of Oenothera has become 
a matter of highly expert judgment and discrimination. In this we have 
certainly traveled far from the viewpoint of the immediately post-Darwinian 
biologists, who, while they were convinced that species come about by 
evolution, were quite willing to despair of ever being able to see evidence 
of the process going on, much less to initiate and control its course and to 
begin to trace out the mechanism of variation. We are confronted with a 
mass of evidence as to species in the making among wild plants, and we no 
longer hesitate to recognize that the breeder can produce, and has from the 
beginning of agricultural science produced, modifications of type quite com¬ 
parable to those which characterize evolutionary species. 
It goes without saying, of course, that from this evolutionary standpoint 
the physiological or biological species which are so common and so sharply 
marked among the parasitic fungi, and the bacterial races differentiated on 
the basis of their specific pathogenicities or cultural characters, are just as 
much to be recognized as evolutionary units as are groups differentiated 
by structural characters. The description and characterization of such 
groups is not easy, but their existence as biologic entities can not be ques¬ 
tioned. The importance from a practical standpoint of their diagnostic 
characters makes sure that their careful and adequate classification will 
always be an attractive field of biological research. 
The consideration of such groups as these leads naturally to the ques¬ 
tion as to the practicability in general of such an evolutionary system of 
classification as I have been arguing for. Will it ever be possible, in any 
considerable number of cases at least, to trace out the devious paths by 
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