Recently at work (a small academic department, absolutely no Emacs support) we upgraded from Emacs version 21.3.1 to 24.3.
In version 21 marking a region with the mouse will put the highlighted text into the kill-ring. Highlighting another region and yanking would insert the previously highlighted text while saving the recent highlight to the kill-ring. You could continue this "alternating replacing" indefinitely.
I cannot recreate this behavior in version 24 even by changing the .emacs like done in this page: https://www.emacswiki.org/emacs/CopyAndPaste
(setq x-select-enable-primary t) (setq x-select-enable-clipboard nil) (delete-selection-mode)
Highlighting the text in version 24 with the mouse seems to just put it into the primary selection but not into the kill-ring until I yank, but then it doesn't produce the inserting alternating text behavior as in version 21, which is desired.
I have also looked into this page https://www.emacswiki.org/emacs/Comments_on_CopyAndPaste but there are many proposed solution and I haven't had the time to see which one would approximate the above mentioned behavior.
Many thanks for your help.