Mail box view window settings

[C. Stark]

Dumb question... recently my mail box view window started splitting when I open mail.(list of message on top and space on bottom to view highlighted message) It never did that before and I want to get it back to where it was before, where all I saw was the email list. I know I can move the bar down but is is annoying to have to do this every time I open mail. I have looked in preferences but can't find anything. Help!