It’s really interesting how they chose what music to track in the second season. After the initial batch of episodes were scored, certain types of episodes would have a lot of music from the same source. For example (and this isn’t all inclusive or without exceptions):
Space based/combat/Starfleet centric episodes would have a lot of music from The Doomsday Machine
(Obsession, The Ultimate Computer, The Immunity Syndrome, The Deadly Years
Episodes with lots of countryside location shooting and primitive tribes would get the similar sounding Friday’s Child/Catspaw
music (A Private Little War
jumps out at me).
Episodes with arena style combat: Amok Time fight music (Gamesters of Triskelion, Bread and Circuses, The Omega Glory)
And the phaser firing finale of The Apple
’s similarities to Who Mourns for Adonias?
Are reinforced by the use of that episodes score from the same scene, just a little differently and enhanced by library cues from The Cage
The other two seasons more freely mixed and matched the music, with less thematic emphasis. For example, a lot of the music from Spock’s Brain
was reused repeatedly in the back half of the third season. The last few episodes opened with the same cue. The first season also just put in whatever seemed right for the moment. However, the second season was interesting in its thematic unity.