As in almost every other facet of science, the diatom taxonomist is constantly faced with the problem of merging phenomena. With the introduction of "quantitative," "mass," or "population" studies on various forms of plant life, lines of distinction between closely related species and varieties have often disappeared, thus indicating a single variable taxon. On the other hand, such studies have upheld the individuality of two or more similar taxa. It is now amply apparent that many of the presently accepted species of diatoms must be re-examined with such concepts in mind, for the purpose of determining, in other than a purely arbitrary fashion, the limits of specific variation. This becomes especially important in group. The most common method is asexual in which case the size of the frustule frequently becomes gradually smaller, until a certain size range is reached. Sometimes other taxonomic features are altered as well. If in a single ecological habitat intergrades are found between two entities, it usually is a good indication that a genetic difference does not actually exist, and that the investigator is dealing with a single entity.