ABSTRACT
The basic problem confronting the phylogenist is one of evaluating the evolutionary changes that are most likely to have occurred. The nature of the pattern is evidenced by present day laboratory and field studies, but is most conclusively demonstrated by groups of organisms whose fossil record is so good that there can be no reasonable doubt about their phylogenetic relationships. One of the basic guiding principles in formulating a phyletic hypothesis is that “plurality is never to be posited without need.” All the reliable taxonomic material available, and its phenetic affinities, should, if possible, be embraced by a single coherent scheme. Economy would be justified solely on the basis that, since taxonomic practice deals in discrete categories, it must tend to misrepresent the continuum of phylal forms that evolution produces. Any phyletic hypothesis must be based primarily on well authenticated, reliably dated and fairly complete fossils alone.
