2io POPULAR. SCIENCE MONTHLY 



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 assump- 

 tion 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 0. mammil- 

 la hi 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 genera- 

 tion, although from such historical data the general trend or direction 

 of succession may be traced. 



