THE SPECIES CONCEPT FROM THE POINT OF VIEW OF 
A PHYSIOLOGIST AND BACTERIOLOGIST 1 
Guilford Reed 
The demands of bacteriology for the classification of a group of organisms 
which on account of their minuteness do not lend themselves to conventional 
structural differentiation has occasioned the development and utilization 
of different criteria of organic relationship. These criteria, though they 
have found little favor outside this restricted field, apply equally to the 
classification of all organisms; and, moreover, from a physiological point 
of view constitute a more fundamental species concept than one based on 
structural differentiation. The fundamental basis of these criteria is the 
conception that organisms differ in the chemical constitution of their 
protoplasm. The methods of making the determination may be considered 
indirect, from chemical standards, but they are none the less precise: 
namely, the methods of immunology. 
I 
It has long been known that, when an animal receives an injection of 
certain substances, antigenic substances, antibodies are developed in re¬ 
action. These antibodies combine in the body of the immunized animal or 
in vitro with their antigens to produce the various familiar antitoxic, lytic, 
agglutinating, precipitating, and many other reactions. From the present 
point of view the significant feature of these reactions is their high degree 
of specificity: an antibody reacts only with its particular antigen and with 
no other. The specificity, moreover, is dependent upon the chemical 
identity of the antigen. Wells and others have shown that when a known 
chemically pure protein is used as an antigen its antibody reacts only with 
that chemically pure protein and with no other protein. Such reactions 
are now finding favor in biological chemistry as a means of identifying 
proteins which can not be distinguished by the ordinary analytical procedure. 
We have, then, in these reactions the most delicate known method of 
detecting chemical differentiation in the complex constituents of protoplasm. 
Application of these methods to the differentiation of bacteria now 
constitutes an extensive and familiar literature. Any detailed consideration 
is out of place; a single example will be sufficient for the present thesis. 
If we have a sample of Bacillus typhosus immune serum, its agglutinins 
will react in high dilution with the organisms used for the immunization, 
1 Read in the symposium on “The Utility of the Species Concept,” at the joint meeting 
of Section G of the American Association for the Advancement of Science, the American 
Phytopathological Society, and the Botanical Society of America, at Toronto, December 28, 
1921. 
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