52 THE CANADIAN ENTOMOLOGIST. 



constantly in the butterflies changed by cold, as related, it is common in 

 this region, predominating over the other varieties. It is also found more 

 or less as far north as New York, though there it is not common. And' 

 moreover its distinctive peculiarity of color is seen in the allied species 

 phao7i, inhabiting the Gulf States, and in vesta, Texas, which in some 

 degree replace tharos in those regions. Both these are seasonally dimor- 

 phic, and both are restricted in the winter form, so far as I can learn, to 

 the single phase denoted by B in tharos. And in their summer gener- 

 ations, both have a close resemblance to the ^mwwwqx tharos, though owing 

 to the increased number of summer generations in the extreme south, 

 permitted by the length of the season, there are phases of the summer 

 form in these species not observable in higher latitudes. It is noteworthy 

 that these two species, the only ones, excepting Batesii, on the Atlantic 

 slope especially near to tharos (and \\\\2X Batesii is, whether it is not 

 another variety of the winter form of tharos, is not yet settled), should be 

 seasonally dimorphic, while of the many other species of the genus 

 belonging to our fauna, not one. so far as is known, shows any marked 

 difterence between its winter and summer generation. 



The significance of these phenomena I take to be this : when phao7i 

 and vesta and tharos were as yet only varieties of one species, the sole 

 coloration was similar to that now common to the three. As they gradu- 

 ally became permanent, or in other words, as these varieties became 

 species, tJiaros was giving rise to several sub-varieties, some of them in 

 time to become distinct and well marked, while the other two, phaon and 

 vesta, remained constant. As the climate moderated and the summer 

 became longer, each species came to have a summer generation ; and in 

 these the resemblance of blood-relationship is still manifest. As the 

 winter generations of each species had been much alike, so the summer 

 generations sprung from them were much alike. 



And if we consider the metropolis of the species tharos, or perhaps 

 the parent species back of that, at the time when it had but one annual 

 generation, to have been somewhere between 37" and 40° on the Atlantic 

 slope, and within which limits all the varieties and sub-varieties of both 

 winter and summer forms of tharos are now found in amazing luxuriance, 

 we can see how it is possible, as the glacial cold receded, that only part 

 of the varieties of the winter form might spread to the northward, and 

 but one of them at last reach the sub-boreal regions, and hold possession 

 to this day as the sole representative of the species. And at a very early 



