"We may all be netted together" - Darwin. This picture is a great way to illustrate and explain the relations/interactions within all animals. Does not necessarily relate to sexual selection, but a good representation of all Darwin theorized of.
The entire library of illustrations, journal entries, and research notes of Charles Darwin has been identified and digitally archived! And it's FREE! http://darwin-online.org.uk/BeagleLibrary/Beagle_Library_Introduction.htm