282 



NATURAL HISTORY. 



the tips of their abdomens. When it settles on the ground, it is difficult 

 to see, as it vibrates in constant motion its tail and wings, so that a 

 mere haze, as it were, exists where it rests. 



The cabbage-butterflies and their allies are distinguished by the preva- 

 lence of some shade of yellow, or even pure white. For many years our 



gardeners were troubled by the depredations of 

 a green worm, which ate the leaves of the cab- 

 bages and turnips, at times causing a consider- 

 able loss. This was the larva of our native 

 cabbage-butterfly, a white insect shaded with 

 scales of dark brown. But a greater evil was 

 to come. Europe furnished us with a new foe 

 to the cabbage, which soon cast the native 

 form into the shade. It is supposed that about 

 ^P^J^)Z£g£™& the year 1857, the larro of the European cab- 



bage-butterfly were introduced into Quebec, 

 and from this centre they have spread all over the northern United 

 States. The larva makes its way to the centre of the cabbage, where it 

 is out of the way of foes and insecticides, and its presence is liable to 

 be unsuspected until the cabbage is cooked. 



In times long past, the whole northern United States was covered with 

 a vast ice-sheet or continental glacier. Its thickness is estimated at nearly 



Fig. 280. — Imported cabbage-butterfly {Pieris rapse) : male, larva, pupa, and female. 



a mile in northern New England. Its southern border has been traced by 

 means of the ' terminal moraine ' — exactly like those of the glaciers of the 

 Alps — from the Atlantic to the Mississippi, and there are abundant traces 

 of it farther west. It crossed Cape Cod, followed the length of Long 

 Island, crossed the northern part of New Jersey and Pennsylvania, and so 

 on through southern Ohio, Indiana, and Illinois. Gradually the climate 

 changed and became warmer, and the glacier melted away; its margin 

 retreated further and further to the north, until at last it entirely disap- 



