fi)llow, as regards size, a law the reverse of that estal)lishc(l by Professor Baird 

 tor the birds and mammals, who shows that they decrease in size southward, 

 though his law of increase in Ihc lengtli ot" certain [)eri|)heral parts wi'sf ward 

 also obtains in the Lopidoplera. '^Piie increase in size westward is, ol edurse, 

 e([nivalent to tiie well-known soulliward increase ol' size in insects; liiougii 

 in a few species of insects, the Colorachtn and Calil'oinian examples are larger 

 than Floridan and Texan insects of the sanu; species. 



Of the iii.sects mentioned in the list, Pliisia hncltpmvartlii is the 

 clearest example (1) ol' the laws of increase in size westward and south- 

 ward ; (2) increase in length ol' peripheral parts westward; (.'>) l)rigiiter, 

 deeper colors westward. 



These facts in the geographical distribution of insects, though they can 

 harilly be called laws until confirmed by a greater nund)er of data drawn 

 from all orders of insects, yet illustrate lo my mind how far climatic variation 

 extends as a lactor in jn-oducing [)rimary dillerences in faun;e within tin- same 

 zone of temperature. Varietal and, in some cases, specific dilferences may 

 have arisen in Asia, Europe, and America from the climatic causes above; 

 stated, but still these were in many cases perhaps inadecpiate in accounting 

 for the present wide distribution of circumpolar species. Here continuity of 

 land, geological as well as meteorological causes, were tiictors. And so, on 

 the other hand, in accounting for the species and types of genera which dis- 

 tinguish faunas in zones of similar temperature, geological causes have been 

 the main factor in their production. For instance, we cannot explain the 

 similarity between tin; insect-fauna ol' the Pacific States and Colorado and 

 that of Eastern Europe and Central Asia without supposing the original 

 migration of the ancestors of the present circmnpolar species from a conunon 

 source, the supposed Tertiary Arctic continent, and the ])reservation of their 

 descendants in their present areas through similar climatic and physical 

 causes. 



ON THE ORIGIN OF THE PRESENT DISTRIBUTION OF THE PITAL.^DNID.E OF NORTH 



AMERICA. 



Having attempted to show that the Phalcenidie of America north of 

 Mexico are composed fundamentally of three elements, viz, of s[)ecies belong- 

 ing to the Tropical American, North Temperate American, and Circumpolar 

 realms, we venture to speculate upon the origin of their present mode ot dis- 

 tribution. That the trojncal .Vinerican forms in our insect-fauna originally 



