B. Tinsley. (2022)cite arxiv:2203.02041Comment: This paper has been transcribed from a hard copy of Beatrice M. Tinsley's original manuscript into a digital file prepared by Michael J. Greener, a PhD student at the University of Nottingham. If you notice any errors or problems with this transcribed paper, please email Michael at either michael.greener@nottingham.ac.uk or mickgreener@protonmail.com. This version 2 includes a clearer Figure 6.