589 



follow, as regards size, a law the reverse of that established by Professor Baird 

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

 though his law of increase in the length of certain peripheral pails westward 

 also obtains in the Lcpidoptera. The increase in size westward is, of course, 

 equivalent to the well-known southward increase of size in insects; though 

 in a few species of insects, the Coloradan and Californian examples are larger 

 than Floridan aud Texan insects of the same species. 



Of the insects mentioned in the list, Plusia hochenwarthi is the 

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

 ward ; (2) increase in length of peripheral parts westward ; (3) brighter, 

 deeper colors westward.. 



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

 hardly be called laws until confirmed by a greater number of data drawn 

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

 extends as a factor in producing primary differences in faunae within the same 

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

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

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

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

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

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

 tinguish fauna? in zones of similar temperature, geological causes have been 

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

 similarity between the insect-fauna of the Pacific States and Colorado and 

 that of Eastern Europe and Central Asia without supposing the original 

 migration of the ancestors of the present circumpolar species from a common 

 source, the supposed Tertiary Arctic continent, and the preservation of their 

 descendants in their present areas through similar climatic and physical 

 causes. 



ON THE ORIGIN OP THE PRESENT DISTRIBUTION OF THE PHAL^ENIDiE OF NORTH 



AMERICA. 



Having attempted to show that the Phalcenidce of America north of 

 Mexico are composed fundamentally of three elements, viz, of species belong- 

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

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

 tribution. That the tropical American forms in our insect-fauna originally 



