HOW BACTERIA ARE NAMED AND IDENTIFIED 21 



to the differentiation of larger natural groups or genera should, in addition to 

 constancy, show some correlation with other constant characteristics. The pres- 

 ence of this relationship or correlation between characters for the division of 

 genera indicates that the groupings are being made along natural rather than 

 artificial lines." 



Type culture. It is quite evident that when a new species of bacterium is de- 

 scribed, it must include the particular culture from which the species description 

 was made. This original culture is termed the type culture. One may develop a 

 definition as follows: A species of bacterium is the type culture or specimen 

 together with all other cultures or specimens regarded by an investigator as 

 sufficiently like the type (or sufficiently closely related to it) to be grouped with 

 it. It is self-evident that different investigators may not draw the same bound- 

 aries for a given species. There are some practical difficulties, but no better 

 definition has been evolved. 



(2) Genus (plural genera). A genus is a group of related species. In some 

 cases a genus may include only a single species (is said to be monotypic); in 

 most cases several to many species are included in a genus. The ciuite pertinent 

 ciuestion should be asked: How close must be the resemblances (how close the 

 relationships) among the species of a group to entitle them to inclusion in the 

 same genus? In other words, how is it possible to delimit accurately the bound- 

 aries of a genus? This is a matter on which there is no agreement, and probably 

 can be none. Much of the confusion in modern bacteriological terminology is to 

 be attributed to this fact. Nevertheless, in the course of time experience tends to 

 delimit many genera with reasonable accuracy. As stated by Hitchcock (De- 

 scriptive Systematic Botany, New York, 1925, p. 9): "Convenience may play 

 a role in determining generic lines. Extremely large groups may be broken up 

 on the basis of differences of smaller degree not common to a group of closely 

 allied species, than if the group consisted of a few species. In general, the botanist, 

 in delimiting genera, keeps in mind two important rec^uirements, that of showing 

 natural aflftnities and that of aiding correct identification." 



However, a genus may be defined helpfully in another way. One of the species 

 described as belonging to a genus is designated as the type species; a genus may 

 therefore be defined as including this type species together with such other 

 species as the investigator (or taxonomist) regards as sufficiently closely related. 

 It is apparent that some authors may draw the lines narrowly, others broadly. 

 Some early authors, for example, recognize only two genera of rod-shaped bac- 

 teria, one for those without endospores {Bacterium), and one for those producing 

 endospores {Bacillus). These genera thus defined are very large, each containing 

 hundreds, perhaps thousands, of species. Other students break up these large 

 genera into many smaller ones. There is not much point to the question as to 

 which is right and which is wrong. A better question is, which is the more con- 

 venient, better represents relationships, better facilitates diagnosis, and proves 

 most useful. 



(3) Family. A family in taxonomy is a group of related genera one of which 



