At the very time that the manuscript of the present paper was ready to be submitted for publication, the senior author, Dr. Jakob Verseveldt, in spite of his age, unexpectedly passed away on 29 March 1987 in his resident town, Zwolle, the Netherlands. Jakob Verseveldt was born on 8 February 1903 in The Hague, the last but one of twelve children (eight sons and four daughters) of the textile-dealer Teunis Jan Verseveldt and Pieternella Vriens. He received his primary and secondary education in his native town and graduated in 1921. In the same year he entered the University of Leiden to study biology and geology. In particular under the guidance of Prof. Dr. P. N. van Kampen, Prof. Dr. E. D. van Oort (both zoology) and Prof. Dr. J. M. Janse (botany), to whom in his own opinion - he owed the most important contributions to his scientific education, he qualified as a biologist (~ M.Sc. degree) in 1927. In the meantime, in 1923, he had begun teaching biology part time at a secondary school (Christelijke Η.Β.S.) in Leiden. In 1926 he also taught in Alphen aan den Rijn, a town situated some 15 km east of Leiden. On 16 April 1927, after obtaining his "M.Sc." he moved to Zwolle in order to take up a full-time teaching post on the staff of the Christelijke Η.Β.S. in that town. This post he was to keep until his retirement on 1 August, 1970. He was apparently exempted from compulsory military service. On 29 May, 1929, some two years after having settled at Zwolle, he married Hendrika de Ruiter, who he had met in The Hague, and who bore him three children: two sons and one daughter. Jakob Verseveldt was a born teacher; he enjoyed and loved teaching, and owing to his natural authority he rarely if ever met with the problems so