Read the poem "City Trees" by Edna St. Vincent Millay. The trees along this city street, Save for the traffic and the trains, Would make a sound as thin and sweet As trees in country lanes. And people standing in their shade Out of a shower, undoubtedly Would hear such music as is made Upon a country tree. Oh, little leaves that are so dumb Against the shrieking city air, I watch you when the wind has come,— I know what sound is there. What is the central idea of this poem?