MICRO-ORGANISMS 31 



two. Therefore, it is necessary to combine them in one large species 

 with the limits defined to include forms found originally in both A 

 and B. 



Thus far only individual micro-organisms as found in nature have 

 been considered. A further problem arises as to manmade forms — 

 mutants and hybrids. Mutants have been produced in many micro- 

 organisms used industrially ; for example, in the production of penicil- 

 lin by Penicillium chrysogenum. Some P. chrysogenum mutants are 

 so unlike their wild- type ancestors that one would never recognize 

 them as P. chrysogenum. Such highly mutated forms probably could 

 not survive in nature if they had to compete with the myriads of 

 other micro-organisms for food. It appears to us that there is really 

 no problem. The mutants should just be given numbers and carried 

 as P. chrysogenum, for we know what their ancestral form was in 

 nature. If, however, the mutant were to be made in nature or to escape 

 into nature and become established ? then it would be treated in the 

 same way as any new taxon found in nature. Undoubtedly mutants 

 have been produced in nature and are now recognized as distinct 

 species. 



The second special category comprises fertile hybrids produced in 

 nature that have become established and are known to have been 

 produced by species A mating with species B to form a fertile hybrid 

 C. Hybrid C has reproduced by itself and become thoroughly estab- 

 lished. The correct manner of citing such an organism is to designate 

 A X B. For example, if Rhizopus arrhizus and Rhizopus stolonifer 

 formed such a hybrid, then the designation would be Rhizopus 

 arrhizus X Rhizopus stolonifer. 



In general, two distinct types or forms of classification will be en- 

 countered. In the first, a purely arbitrary system is developed with- 

 out any regard to natural relationships. It is purely mechanical. 

 For example, F. E. Clements and C. L. Shear in their book, The Genera 

 of Fungi, separated the families of the order Moniliales as to whether 

 the fungi had either light-colored spores (hyaline) and mycelium or 

 dark spores and mycelium. The light-colored forms were placed in 

 families divided on the basis of whether the conidia were one-celled, 

 two-celled, and so on. Obviously, very closely related forms might 

 have both one- and two-celled spores ; yet in this type of system they 

 are not in the same genus or even in the same family. A similar me- 

 chanical key to the genera of bacteria is given in Bergey's manual (4) . 



The second type of classification is based on natural relationships. 

 Thus, the families of the order Mucorales {H) are erected on the basis 

 of genera having exactly the same type of sexual reproductive organs 

 and similar types of asexual reproduction. The genera Zygorhynch us 

 and Mucor have sporangia exactly alike and the sexual stage, zygo- 

 spore, is represented by a roughened thick-walled spore. The distinc- 

 tion between the two genera is based on the homothallic condition and 

 on the fact that one of the progametangia is larger than its mate in 

 Zygorhynchus. In contrast, most species of Mucor are heterothallic, 

 and its few homothallic species have progametangia alike in size and 

 appearance. In some taxonomic treatments both types of keys for 

 identification are incorporated, so that one can use one key to check 

 against the other. 



