74 THE CLASS OF INSECTS. 



"The western district is the maritime slope of the continent 

 to the Pacific, and thus includes California, Oregon, and Wash- 

 ington Territories. 



"These great districts are divided into a number of prov- 

 inces, of unequal size, and which are limited by changes in 

 climate, and therefore sometimes distinctly, sometimes vaguely 

 defined." 



"The method of distribution of species in the Atlantic and 

 Pacific districts, as already observed by me in various memoirs, 

 is entirely different. In the Atlantic district, a large number 

 of species are distributed over a large extent of country ; many 

 species are of rare occurrence, and in passing over a distance 

 of several hundred miles, but small variation will be found in 

 the species obtained. In the Pacific district, a small number- 

 of species are confined to a small region of country ; most 

 species occur in considerable numbers, and in travelling even 

 one hundred miles, it is found that the most abundant species 

 are replaced by others, in many instances very similar to them ; 

 these small centres of distribution can be limited only after 

 careful collections have been made at a great number of locali- 

 ties, and it is to be hoped that this very interesting and im- 

 portant subject of investigation may soon receive proper atten- 

 tion from the lovers of science of our Pacific shores. 



" In the Central district, consisting, as it does to a very 

 large extent, of deserts, the distribution seems to be of a mod- 

 erate number of species over a large extent of country, with a 

 considerable admixture of local species ; such at least seems to 

 be the result of observations in Kansas, Upper Texas, and 

 Arizona." 



There are a very few species which range from New England 

 to Brazil, and fewer still (Xyleutes robinice, according to Bois- 

 duval, is found in California) range from New England to 

 California. Junonia ccem'a, according to authors, is found both 

 in the Southern States and California, and Pyrrliarctia Isabella 

 of the Eastern States would be easily confounded with P. Cali- 

 fornica. 



Variation. Islands afford more variable forms than conti- 

 nents ; the Madeiran insects and those of Great Britain vary 

 more than the same species found on the continent of Europe. 



