1922] HELLER—ANAEROBIC BACTERIA ps 
in these groups require more study, and other and better ways 
may be found to divide them. The following general rules will 
probably be found convenient for classifying bacteria. 
BioTyPE.—Strains that differ from each other in characters that 
are readily subject to mutation, and that breed true, may be termed 
biotypes. The word subspecies has so long been used in the higher 
groups with a geographical connotation that it will not be well to 
use it for subdivisions of bacterial species. The term type may 
then well be left as an independent unit of our vocabularies for 
non-specific use. 
SPECIES.—Strains that behave alike in those characters that 
within their genus have not been found to mutate readily, may be 
grouped as species. The occasional derivation of one species from 
another is no more to be considered impossible than it is in 
higher groups. Bacteriologists have too long considered the 
species conception in higher groups as one of fixed immutable 
orms, whichit isnot. The recombination effects noted by DE Vries 
(47), in which he showed the independent origin of three well defined 
types (Oenothera nanella, O. eliptica, and O. lata) from two others of 
quite widely divergent character (O. Lamarckiana and O. laevifolia) 
apparently cannot occur among bacteria. These recombination 
effects allowed the sudden appearance of groups of mutations that 
had occurred previously. Among bacteria, however, because of 
the rapidity with which vast numbers may be bred and the energy 
with which selection acts, several characters may be changed nearly 
simultaneously, and similar effects to those noted by DE VRIES may 
occasionally be observed, namely, the appearance of a number of 
new characters within a short space of time. On analogy, it would 
be perfectly reasonable to describe the strains that result from 
various changes as separate species. It is quibbling to define the 
word “species” so closely that no elasticity should be allowed in 
its application. Our knowledge is too meager and the possibilities 
too great to restrict closely the meanings of taxonomic words. 
Changes that may be considered specific may be discovered or 
perhaps even brought about by treatment of bacteria that is more 
radical than anything tried with plants, and there is no reason why 
