WITH FURTHER HISTORIES. 161 



gions, and are rare in alpine and subarctic dis- 

 tricts. As we go north, the colors become less 

 sharply defined, then gradually fade away or be- 

 come blended with surrounding tints ; the red 

 first disappears, the blue follows, the yellow 

 longest maintaining its hold, although Prittwitz 

 considers the blue the most persistent. As soon 

 as we leave the tropics these brighter colors are 

 seldom seen in combination ; and as we approach 

 the higher temperate regions, we are constantly 

 struck by the impurity of the tints. Take a 

 single example from the common sulphur butter- 

 flies of the genus Eurymus [see Figs. 102, 103] ; 

 the more southern species have the under surface 

 of the hind wings of a clear canary yellow, and 

 what few spots they possess are clustered into 

 sharply defined markings next the margin ; these 

 features predominate until we arrive at the Middle 

 States, when a change begins ; and on reaching 

 Labrador we find the opposite extreme, a ground 

 color of greenish yellow completely flecked with 

 atoms of brown, giving the wing a grimy appear- 

 ance ; while the marginal markings are simply 

 more densely clustered atoms, forming spots 

 which gradually pass into the general dinginess 

 of the wing. 



This loss of purity and greater or less suffusion 

 of markings is characteristic of northern and 

 alpine forms, and is in perfect accord with another 



