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- 
