Geographical Distribution. 



Leaf hoppers are so well distributed over the earth that they 

 are truly cosmopolitan. They are well represented in all the 

 faunal realms, and in some countries are among the commonest 

 of the insects. In his catalogue of the Hemiptera of America 

 north of Mexico, published in 1917, Mr. Van Duzee lists about 

 700 species, and the number now known must be well beyond 

 that. I have been unable to get any estimate of the total 

 number of species known to science. 



Professor Osborn has pointed out two facts of great in- 

 terest when one views this group as a whole or when the 

 fauna of two continents are compared. First, the fact is soon 

 observed, that the leaf hopper fauna of even two widely- 

 separated portions of the earth, are essentially and funda- 

 mentally alike in group characters. This is taken as showing 

 a common origin of the groups. And second, that though in 

 the main characters and larger groupings there are so many 

 similarities, yet there seem to be relatively very few cases of 

 specific identity between the species of such separated coun- 

 tries or continents. Examples of this fact are numerous when 

 our own forms are compared with the European. The sub- 

 family Paropinas, for example, occurs on both continents, yet 

 not one of our eight species seems to occur in Europe. Of our 

 seventy-five or more members of the genus Deltocephalus only 

 four are known to occur in Europe. And this is about the case 

 in almost any group one may choose. 



This fact would argue for an early separation of our forms 

 from the European and for a consequently long development 

 here. It would seem to indicate also that introduction of leaf 

 hoppers into new continents, separated by oceans, is to-day 

 rather rare if occurring at all. And when one considers the 

 few adaptations of these forms for transmission, especially as 

 to life history, one is all the more convinced that such intro- 

 duction does not often take place. If such be the case it is 

 evident that the distribution of the leaf hoppers over the earth 

 must have occurred in the early times when the different por- 

 tions of the earth were more connected than they are now. 



(:^0) 



