No longer getting separate full message text in Mail viewer window only list of messages column

Suddenly, for no apparent reason, when I open Mail the viewer window shows only the list of mailboxes, headed by the Inbox, and the list of messages indicating their senders. The actual full text message no longer appears to the right of the sender when I scroll down the list of senders.

I can turn the list of mail boxes on and off and I can extend the right hand edge of the list of senders until the message appears under sender's name but this is not what I want. I want the normal viewer window back.

Have tried every option that seems relevant in Mail Preferences and under View in the Mail menu bar but nothing has the desired effect.

How can I get back to the normal Mail view window with list of mailboxes, senders and text message side by side?