Dedicated — with deep gratitude — to Professor Dr. Dietrich Schneider

Supported by the Deutsche Forschungsgemeinschaft (SFB 4/B6). Stimulating criticism by various colleagues, particularly by H. Sass, is thankfully acknowledged as well as working out of Fig. 7 by A.W.R. McCrae — he and R.I. Vane-Wright greatly improved the English of the manuscript for which I am deeply indebted, too.