THE UNDERLYING IDEA 65 



of the long-suffering shade trees. This caterpillar is 

 covered with bristling hairs, very closely set. Almost 

 any bird objects to hair in his victuals; and this par- 

 ticular larva has hair more than ordinarily objection- 

 able, for it irritates wherever it pricks the sensitive 

 skin. This coating seems to protect the caterpillar 

 from the sparrow, with the result that Philadelphia's 

 trees were soon nearly defoliated by this comparatively 

 new pest, worse than the spanworm. With the pav- 

 ing of the city's highways and the consequent shut- 

 ting off of the air from the roots, the trees have 

 largely disappeared from the streets of Philadelphia. 

 With them have gone a fair portion of the tussock 

 worms, but the sparrow holds his own. Here is a new 

 bird in the field, and the struggle for existence on 

 the part of every other kind of bird is now more com- 

 plicated and severe. The sparrow can live where the 

 rest of the birds have no possible chance. He throve 

 so well in this country that by 1875 he had spread 

 over five hundred square miles in the neighborhood 

 of our larger Eastern cities. Thus far almost every- 

 body was pleased with the new introduction. Within 

 the next five years he had spread over more than fifteen 

 thousand square miles, and wise men were beginning 

 to feel doubtful of the virtues of their aforetime 

 friend. When by 1885 more than five hundred thou- 

 sand square miles had been occupied by the enterpris- 



