geography and floristics. That the actual mechanical contiguity of two forms competing for the same conditions of habitat would result in some stress is to be taken for granted, but this vegetative struggle would by no means be severe enough in any case to eliminate one from any region. If the advocates of the idea that closely related species do not occupy the same region take this ground on the assumption that hybridization disturbances would follow, here again would be an unwarranted assumption. Readiness of hybridization is by no means a measure of consanguinity, and any slight difference of habit, so small perhaps as not to be capable of description, might ensure pure fertilization. In the case of forms differing by one or a few characters, Mendelian splitting might operate to maintain the forms even if hybridization did occur.
The accompanying photograph of Opuntia fulgida and O. mammillata presents two forms so closely related that the latter has been taken as a variety of the former by some botanists, but it has been found to be a distinct and physiologically unified strain, and worthy of specific rank. These two forms are widely intermingled, though of course not many instances of actual contact such as appears in the illustration are to be found. The flora in the vicinity of the Desert Laboratory, at Tucson, presents scores of similar examples among other species representing many genera.
Aside from such misinterpretations, a prolific source of confusion lies in the widely different conceptions as to the nature of the taxonomic units used in zoological and botanical writings, as a consequence of which we have some zoologists calling attention to the supposed fact that certain botanists of differing views have no real conception as to the nature of 'species' and 'varieties.' Such statements serve the useful purpose of emphasizing the disadvantageous prejudices under which their authors labor.
While such misunderstandings contribute to hinder progress and confuse the subjects, the basal and underlying fault consists in the fact that taxonomic and geographic methods are not in themselves, or conjointly, adequate for the analysis, or solution of genetic problems. The inventor did not reach the solution of the problem of construction of a typesetting machine by studying the structure of printed pages, but by actual experimentation with mechanisms, using printed pages only as a record of his success. Likewise no amount of consideration of fossils, herbarium specimens, dried skins, skulls or fish in alcohol may give any actual proof as to the mechanism and action of heredity in transmitting qualities and characters from generation to generation, although from such historical data the general trend or direction of succession may be traced.
