44 BIOLOGICAL LECTURES. 



exposure should be such as to advance the transition flora 

 about 300 meters higher along the northern wall than along 

 the southern. As a matter of fact, the effect of slope expo- 

 sure is totally lost, and the boundary of the transition zone is 

 pushed backward and downward 300 meters below its normal 

 level. 



Professor Tyler-Townsend states that similar illustration of 

 this principle is to be seen in the Mesilla valley, a portion of 

 the valley of the Rio Grande in New Mexico. The valley has 

 an elevation of 1200 meters and rises gradually on the east to 

 an elevation of 1700 meters, on the slopes of the Organ Moun- 

 tains fifteen kilometers distant. As a consequence of the 

 meteorological phenomena detailed above, Larrea tridentata 

 advances northward along the mesa above elevations of 1300 

 meters, while it cannot grow in the valley 100 meters below. 

 At the same locality he reports the presence of a peculiar fly, 

 Raphiomidas xanthos Towns., of the lower Sonoran fauna, not 

 found elsewhere north of Lower California. Volucella opales- 

 cens Towns., a member of the subtropical group of the genus, 

 and Lecanitim imbricatum, a distinctly neotropical type, also 

 occur in this locality. Vohicella haagii Jacun. occurs on the 

 mesa in question, as well as in similar locations in the White 

 and Magdalena mountains, where it is beyond the general curve 

 of its zonal limits. 



That this distribution is entirely the result of the meteor- 

 ological factors, chiefly inversions of temperature and cold- 

 air drainage, seems to be indicated by the fact that genera of 

 flies such as Gymnosoma y Hyalomyia, Trichopoda, Cistogaster, 

 Hemyda, and Ocyptera, which range from the transition zone 

 to the tropics, are found both on the mesa, along with the other 

 subtropical forms, and also in the valley, showing that food sup- 

 ply or other conditions of environment do not account for the 

 northward advance of the southern forms enumerated. 



The facts which have been brought before the writer in the 

 investigations described above lead to the belief that many of 

 the so-called aberrations of distribution which have been re- 

 corded in various parts of the world may be found directly 

 attributable to the influence of the ascending air currents, 



