THE BUTTERFLIES AND MOTHS 49 



Hampshire and the Rocky Mountains in Colorado. As will 

 be seen later, the geological period immediately preceding the 

 present was an age of ice, in which the North American conti- 

 nent was covered by ice as far south as an irregular line drawn 

 through New York, Pennsylvania, Missouri, South Dakota, 

 and Oregon, and arctic conditions existed over the rest of 

 the continent. At this time these butterflies were probably 

 distributed over the country. As the ice slowly melted and 

 the climate became warmer, they found only on the summits 

 of these widely separated mountain ranges the conditions to 

 which they had been accustomed, all those occupying the 

 region between being driven to the northward or destroyed 

 by the heat. 



Swallow-Tail Butterflies. The magnificent insects called 

 swallow-tail butterflies, widely distributed over the world, 

 have received their common name on account of the prolon- 

 gation of the posterior wings. They are especially interest- 

 ing to the naturalist, not only for the variety and beauty of 

 their colors and the elegance of their form, but also because 

 they often exhibit, within the limits of a single species, great 

 variation in color, size, and even in the shape of the wings. 

 The variation in form, size, and color between individuals of 

 the same species is spoken of as dimorphism (Gr. di, two ; 

 morphe, form), if the variations show two well-marked types ; 

 and as polymorphism (Gr. poly, many), if there are several 

 different variations. In many species the males differ greatly 

 from the females ; this is called sexual dimorphism. Many ex- 

 amples are known of broods of the same butterfly appearing 

 at two different seasons. These broods often differ widely in 

 size and color markings. This is known as seasonal dimor- 

 phism. In some cases seasonal dimorphism and polymorphism 

 seem to result from conditions of temperature. A swallow- 

 tail butterfly (Papil'io a'jax), which is widely distributed in the 

 eastern United States, has, in the latitude of West Virginia, 



