88 



trate my points. To have done so would have required a greater knowledge 

 of local distribution of species than we now possess. What is at present 

 especially needed is intelligent, continuous, persistent local collecting, 

 such as is heing done by Mr. Evans, of Evansville, Profs. Blatchley, of 

 Terre Haute, and W. P. Shannon, of Greensburg, and Judge McBride and 

 sons, of Elkhart. It is only by long acquaintance with a locality that 

 we become familiar with its fluctuating insect fauna — species that do not 

 occur every year, and when they do appear are present only in scant 

 numbers and over a limited area. 



A careful study of species, other than those here given, may throw much 

 light on the problem of general geographical distribution, and our divid- 

 ing line is supposed to be correct in a general way, as, of course, there can 

 be no such thing as an exact or continuous line of demarkation. This 

 will of necessity be more or less irregular. Again, a species spreads over 

 an area particularly adapted for its occupancy. But, no sooner is this 

 done than the individuals along the frontier begin to adapt themselves to 

 an environment but slightly unfavorable, and, as their adaptation changes, 

 so do they slowly advance outward from the territory originally occupied. 

 A series of to them favorable seasons might occasion the occupation of a 

 wide margin of adjoining country, while a series of unfavorable seasons 

 might sweep this tide of advance back nearly or quite to the place of its 

 origin. But, as the receding tide of the ocean leaves many pools of water 

 in the depressions of rock, so will there be left, in especially favorable 

 nooks, a few of the insects which will retain their hold and form small, 

 local colonies, of perhaps not more than a few individuals, and the off- 

 spring of these will meet the investigator long distances from the real 

 habitat of the species. There is scarcely a collector who does not know 

 of one or more small, secluded areas, in his neighborhood, that are rich in 

 varieties, and which he seldom visits without satisfaction, and frequently 

 he is astonished at his success. How long this ebb and flow has been 

 going on, and how many species have been brought to us in this way, are 

 problems we are yet unable to solve. Therefore, these facts have been 

 brought together, and are here presented, not as a finished, or, indeed, as 

 an advanced study, but rather as a primary outline, to be revised and 

 modified as our knowledge of the geographical distribution of our species 

 shall be enlarged by additional study and research. 



