May, 1923] 
STAKMAN — SPECIES CONCEPT 
2 4 I 
been used as a basis for the multiplication of one true morphologic species 
into several imaginary ones. And, on the other hand, several distinct 
species sometimes have been described as a single species, either on account 
of faulty technique or on account of the fact that the range of conditions 
under which the fungi were studied was not sufficiently wide to make 
possible the detection of differences which become apparent only under 
certain environmental influences. The range of variability of many species 
of fungi is very wide. The character of growth, the ability to reproduce, 
and the morphology of the organism may be influenced profoundly by the 
amount and kind of food available and by environmental conditions. 
Klebs, Thom, Coons, Duggar and his associates, and other investigators 
have demonstrated this conclusively. 
The pathologist is vitally interested in knowing the morphology of a 
pathogenic organism, not only on one host and under one set of physico¬ 
chemical conditions, but under all possible conditions. And he is especially 
concerned with the question as to whether essential morphologic identity 
means also essential physiologic identity. In fact he knows that in many 
species it does not. 
Every one now knows that there may be physiologic races within a j 
morphologic species, and there has been a growing tendency, therefore, to 
use physiologic characters for the delimitation of species. The bacteria in 
general are separated into species on the basis of their physiological reac¬ 
tions, and very little objection is raised. There also is a tendency to use 
physiologic characters more and more in systematic work on fungi. The 
taxonomic work of Appel and Wollenweber, and of Sherbakoff on Fusarium, 
and of Thom on Penicillium was based not only on morphologic, but also 
on cultural or physiologic characters. More and more the description of 
species is being based on material grown on standard media or on several 
hosts, and under known conditions. Unless this is done, descriptions often 
mean nothing, because the so-called species may contain not only several 
morphologic, but also several physiologic, races. 
There seem to be different degrees of specialization into biologic forms, 
specialized races, chemical species, Gewohnheitsrassen, physiologic races, or 
whatever one chooses to call them. These terms were not all used exactly 
synonymously originally, but, since the differences represented by the terms 
seem to be in degree rather than in kind, they are all called biologic forms 
in this paper. These forms supposedly are practically indistinguishable 
morphologically, although slight differences are known to occur; but they 
differ decidedly from each other in their physiologic action. There would j 
seem to be several classes of such forms, although the categories into which 
they can be placed may represent no really fundamental differences. 
Dox has shown that there may be such distinct chemical differences 
between species of Penicillium and Aspergillus that the species can be 
recognized more easily by chemical than by morphologic characteristics. 
