Furthermore, most artificial media are slight variations on a few common and simple themes, and are very selective, permitting the growth of a moiety only of the fungi present. In addition, many fungi grow so slowly that they are overwhelmed by the more rapidly germinating or spreading forms, or on the other hand, they may be eliminated by the metabolic products of different adjacent colonies. The extremely selective nature of the technique commonly used is shown if one tabulates systematically all the fungi which have been recorded or described in soil investigations. Of _Phycomycetes_ there are fifty-six species of eleven genera; of _Ascomycetes_ twelve species of eight genera; and of _Fungi imperfecti_, including _Actinomycetes_ but not sterile _Mycelia_, 197 species of sixty-two genera. Rusts and Smuts one might not expect, but that of the multitudes of _Basidiomycetes_ growing in wood and meadow not one should have been recorded is indeed startling. It was at first thought that many imperfect fungi might be conidial stages of _Basidiomycetes_, but much search among forms isolated at Rothamsted has, up to the present, failed to reveal clamp connections in the hyphæ. Since various species of soil fungi have different optimum temperature, humidity and other conditions[3] one would not expect to find an even geographic distribution. Very little is yet known of this aspect, but _Rhizopus nigricans_, _Mucor racemosus_, _Zygorrhynchus vuilleminii_, _Aspergillus niger_, _Trichoderma koningi_, _Cladosporium herbarum_, and many species of _Aspergillus_, _Penicillium_, _Fusarium_, _Alternaria_, and _Cephalosporium_ have been commonly found throughout North America and Europe wherever soils have been examined. Species of _Aspergillus_, however, would appear to be more common in the soils of south temperate regions and species of _Penicillium_, _Mucor_, _Trichoderma_, and _Fusarium_ more abundant in northern soils. It is well known that in many plant and animal communities there occurs a definite rhythm, various species following each other in a regular sequence as dominants in the population. Although it is not yet possible to make any definite statement there would seem indications that this may also be true of the soil fungi.