46 MANUAL OF DETERMINATIVE BACTERIOLOGY 



strain of influenza vims differs from the B strain in certain ways. In 

 other words, "strain" is not a synonym of "type" or "variety". We may 

 have as many yellow strains of the typhoid bacillus as we have of cultures 

 of it, from different sources or specimens. 



(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 ques- 

 tion asked above may be paraphrased. 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 de- 

 limit accurately the boundaries 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 course of time experience tends to delimit many genera 

 with reasonable accuracy. As stated by Hitchcock (Descriptive Sys- 

 tematic 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 requirements, 

 that of showing natural affinities 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 be defined then, 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 authors, for example, recognize only two 

 genera of rod-shaped Ijacteria, one for those without endospores {Bac- 

 terium), 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 convenient, better 

 represents relationships, better facilitates diagnosis and proves most useful. 

 As organisms become better known, it may be possible through the agency 

 of the International Committee on Nomenclature to reach agreements 

 where lack of agreement leads to serious confusion or misunderstanding. 



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

 general the name of the family is made from the name or former name of 

 one of their genera by affixing the suffix -aceae to the root. The word is 

 regarded as plural. Among bacterial families commonly recognized are 

 Bacillaceae, Bacteriaceae, Micrococcaceae, Spirochaetaceae, Actinomycetaceae 



