Believe it or not, Darwin didn't study sexual selection that much. He didn't write about it because he didn't know about it- he didn't assert a knowledge that wasn't there. He did take note that some barnicle species that he orginaly thought to be A-sexual, did have males, they were just microscopic and basicaly spent their life as a sperm factory. I did learn this stuff from a college course in evolutionary biology that I took last quarter. If you would like to dispute my thoughts, here's an easy read: