1. Acknowledgments. We are indebted to the following persons who sent living material: Dr K. Aagard, D. Benyamini, F. Catzeflis, U. Eitschberger, J. Forsberg, Prof. R. Hauser, T. Hayasaka, P. Jaksic, Dr F. Kasy, Dr H. Katakura, Dr D. Murphy, Dr T. Racheli, J. Reichel, Prof. A. M. Shapiro, Dr G. Thomson, Prof. P. Vogel and O. Yoshiaki and others which are named in earlier publications. Comments and suggestions of Prof. A. M. Shapiro and D. C. Webb considerably helped to improve the manuscript. This study has been supported by grants from the Swiss National Science Foundation (No. 3.640.80), Hochschulstiftung der Universität Bern and Dr Karl Bretscher-Stiftung Bern. We acknowledge the technical assistance of Mrs L. Frauchiger and V. Siegfried. Prof. R. Hauser continuously encouraged these studies.
2. Avise, J. C., Systematic value of electrophoretic data. Syst. Zool.23 (1974) 465–481.
3. Ayala, F. J., Tracey, C. A., Hedgecook, D., and Richmond, R. C., Genetic differentiation during speciation process inDrosophila. Evolution28 (1974) 576–592.
4. Bernardi, G., Revision de la classification des espèces holaretiques des genresPieris Schr. etPontia Fabr. Miscenea ent.44 (1947) 65–79.
5. Bowden, S. R., Hybrids within the EuropeanPieris napi L. species group, Proc. S. Lond. ent. nat. Hist. Soc. (1956) 135–159.