If you're talking about where the flute first enters, that theme begins as a pentatonic scale, more or less. You can easily demonstrate that scale to yourself by playing only the black keys on a piano. It's the scale that's generally associated with Asiatic music. It then morphs into a diatonic idiom - mostly in major - with some chromatic (half-step) neighbor tones thrown in. In other words, it becomes the type of melody that one could easily identify as being 'late romantic' or late 19th century. It's beautiful, though, isn't it? Mahler simply doesn't get enough credit as a melodist (writer of melodies).
