TAXONOMY Of MUSCOIDfcAN FLIES — TOWNSEND I 5 



atmospheric pressure, food, etc. — in other words, by the pressure of 

 environment, which means all . stimuli taken together and acting 

 together. 



It is thus seen that climatic or meteorologic conditions are potent 

 factors in the evolution of forms of life, and that as a rule one form 

 does not inhabit two widely different life zones or areas. Few, if 

 any, forms inhabit both temperate and tropical regions, or both 

 humid and arid regions. The external stimuli natural to the differ- 

 ent zones and areas result in the modification of forms coming within 

 the sphere of their influence, and the consequent production of new 

 forms. Thus the progeny of individuals of one and the same form, 

 spreading gradually through areas where they become subjected to 

 new sets of stimuli, are gradually differentiated into distinct forms 

 through the pressure of environment. Dr. Merriam's exposition of 

 this law in his address before Section F of the American Association 

 for the Advancement of Science, at its 55th meeting (Proc. Am. Ass. 

 Adv. Sci., 1906, pp. 387-0), is an admirable one, and can be studied 

 with much profit. His observations, as there given, agree perfectly 

 with the results of the writer's studies in Diptera. For instance, the 

 arid and humid regions of North America will be found to possess 

 very few species in common. These very different life areas are 

 divided and subdivided by temperature as we go north or south, or 

 ascend above sea level, and again and again subdivided by various 

 climatic and other environmental factors. The result is many sepa- 

 rate life areas, more or less restricted, each of which exhibits a dis- 

 tinct stamp of environment. Intergradations occur along the periph- 

 eries of the ranges of closely related species, when such lie contig- 

 uous, as they often do. These intergradants must not be confused 

 with the normal specimens of the form as exhibited throughout the 

 more central portions of the area of range. That the intergradants 

 occur between two such forms does not invalidate the distinctness of 

 the forms themselves. 



It may safely be stated as a theorem in bionomics that, given an 

 arid area and a humid area contiguous to each other, both originally 

 stocked with individuals of the same form — whether of Diptera or 

 any other order of life — the descendants of this form will not remain 

 identical in the two areas throughout any considerable period of 

 time. The theorem may be enlarged to include temperate and trop- 

 ical contiguous areas, and many divisions and subdivisions of these 

 and of the arid and humid areas as well. The resultant differentia- 

 tion is brought about by dynamic variation, incited by the respective 

 sets of external stimuli acting on the germ plasm of the ova con- 



