l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



tained within pregnant females of the form, already referred to as 

 demonstrated by Tower. 



A careful study of these factors and of the results produced by 

 them demonstrates the fallacy of the idea that forms from the north 

 Atlantic coast region of the United States and the south Gulf coast 

 region of Mexico are identical. In other words, forms originally 

 described from Vera Cruz are not to be identified in Massachusetts 

 material. Likewise, forms from arid regions are not to be identified 

 in humid region material. Furthermore, European species are not 

 to be identified in American material, except in the few cases of 

 forms that have been imported through the agency of man. There 

 exist today practically no Muscoidean forms common originally to 

 Europe and North America. The Muscoidea did not originate from 

 circumpolar stock. The forms that immigrated to northern America 

 from Eurasia during the warm periods that existed in the subarctic 

 region in interglacial times have long since given rise to new forms, 

 and no longer persist in their original state. 



There are certain more or less cosmopolitan flies, such as Musca 

 domestica, Stomoxys calcitrans, Luciiia ccusar, Calliphora erythro- 

 cephala, and others, which find their natural environment in the wake 

 of man. These are not so amenable to the above factors, but even 

 they show some effects of their agency. A considerable number of 

 such species doubtless accompanied primitive man in his wanderings 

 through various parts of the earth. Other species are of compara- 

 tively recent dispersion through commercial agencies. Both classes 

 have been involuntarily spread by man. The detection of the second 

 class calls for extremely careful study ajid fine powers of perception. 

 Still another and very recent class has been purposely spread by man 

 for economic ends. 



A word may be said as to the difficulty of distinguishing between 

 many of the distinct but closely similar forms that occur in the Mus- 

 coidea. While many of these forms that closely resemble each other 

 do so by virtue of their close relationship through common origin, it 

 is evident that others of more diverse origin have developed a close 

 resemblance through counterfeitism 1 attained by means of natural 



1 The writer herewith proposes a change from the use of the words mimic, 

 model, and mimicry. The terms "mimic" and "model" have nothing", except 

 usage and priority, to commend them. "Mimic" is exceptionally faulty, and 

 does not nearly convey the intended meaning. In the strict sense of the word 

 a mimic is one who, by sound or action, imitates another. The word does not 

 imply any idea of form, color, or size. The word "counterfeit," however, 

 embodies the full concept. Again, "model" does not carry the idea of size, 

 and in an art sense only partially that of form; moreover, it is not necessarily 



