312 



SCIENCE. 



[N.S. Vol. XXIII. No. 582. 



America, from the Gulf of Mexico to Alaska, 

 there are no abrupt and insuperable barriers, 

 either topographic or climatic, to the continu- 

 ous distribution of many forms of life; the 

 diversity of conditions, due primarily to cli- 

 mate, however, is so great that few, if any, 

 species of mammals range throughout this 

 whole area, or of birds that have a breeding 

 range of this great extent. Each climatic 

 zone has its peculiar associations of life, made 

 up by the overlapping of the ranges of dif- 

 ferent sets of species, whose final boundaries 

 are formed for each by a particular combina- 

 tion of climatic conditions. Aside from the 

 temperature zones, just considered, other cli- 

 matic factors, as especially that of rainfall, 

 become active in passing from the eastern 

 border of the United States westward to the 

 Rocky Mountains and the Pacific coast. 

 There thus arise a large number of faunal 

 areas aside from those dependent on zones of 

 temperature. The transition between these 

 also is not sufficiently abrupt, except where 

 locally complicated with topographic barriers, 

 to prevent the continuous distribution of many 

 species of birds and mammals. But the transi- 

 tion at certain points between contiguous 

 forms is much more rapid over certain com- 

 paratively narrow belts than elsewhere. If 

 we take some central point in the eastern 

 United States, as for instance Columbus, Ohio, 

 we may go east, west, north or south for sev- 

 eral hundred miles in an area where the 

 amount of local or climatic differentiation is 

 so small as to be practically indistinguishable; 

 in other words, we are in the central portion 

 of a large area where the conditions of life 

 are comparatively uniform, and are reflected in 

 the practically constant characters — color, 

 size, etc. — of its animals. If, however, we 

 pass westward to about the ninety-eighth 

 meridian, on the same parallel, we meet with 

 wholly different conditions; we are then in a 

 transition belt, where the characters of the 

 animals are unstable; we have left the eastern 

 phases of many of the mammals that range 

 continuously westward, but have not yet 

 reached the Great Plains phases that come in 

 and for a long distance take their place as 

 stable western forms representing the equally 



stable eastern forms we have left behind. We 

 are in a comparatively narrow belt of inter- 

 mediates — in some respects the hete noir of 

 the systematist, in others constituting an in- 

 valuable key to otherwise intricate problems 

 in evolution — which in turn reflect the action 

 of intermediate conditions of environment 

 between two well-marked areas. There is no 

 barrier, topographic, climatic, or even bio- 

 nomic, under any reasonable use of these 

 terms; the transition belt is narrow, seldom 

 more than thirty to fifty or a hundred miles 

 in width ; there is every opportunity for inter- 

 breeding, and no barrier other than the seden- 

 tary disposition of individual animals. If 

 this fulfills Professor Ortmann's definition of 

 ' no continuity of bionomic conditions,' and 

 meets President Jordan's definition of ' isola- 

 tion,' we at least understand each other. 



J. A. Allen. 

 Amebican Museum of Natural Histoby, 

 New Yobk City, 

 January 21, 1906. 



SPECIAL ARTICLES. 



ON THE BREAKING-UP OF THE OLD GENUS CULEX. 



Within the past two years attempts have 

 been made to break up the old genus Culex in- 

 to smaller genera based chiefly or wholly upon 

 the structure of the claspers of the male. 

 That too great stress has been laid upon this 

 character in many cases is the opinion of more 

 than one systematic worker in the Diptera. 

 Thus such very closely related species as 

 sylvestris and cantaior are separated into dis- 

 tinct genera, while, on the other hand, such 

 very diverse forms as sollicitans, squamiger, 

 higotii, annulatus, janitor and discolor are 

 placed in one and the same genus. In this as 

 in other cases, any attempt at a classification 

 founded upon a single character is certain to 

 produce unsatisfactory results ; only by taking 

 into consideration the habits and entire life 

 cycle of the various forms can anything ap- 

 proximating a natural grouping be formu- 

 lated. 



The writer has recently been able to make 

 a long-contemplated study of the species of 

 this country placed by Theobald in the genus 

 Culex in the first two volumes of his mono- 



