The first volume of Evolutionary Biology was published twelve years ago. Since that time twelve volumes and one supplement have appeared. As stated in earlier prefaces, we are continuing the focus of this series on critical reviews, commentaries, original papers, and controversies in evolu tionary biology. It is our aim to publish papers primarily of greater length than normally published by society journals and quarterlies. We therefore invite colleagues to submit chapters that fall within the focus and standards of Evolutionary Biology. Manuscripts should be sent to anyone of the following: Max K. Hecht, Biology Department, Queens College of the City University of New York, Flushing, New York 11367; William C. Steere, New York Botanical Garden, Bronx, New York 10458; Bruce Wallace, Department of Genetics, Cornell University, Ithaca, New York 14850. The Editors vii Contents 1. Apparent Changes in the Diversity of Fossil Plants: A Preliminary Assessment Karl J. Niklas, Bruce H. Tiffney, and Andrew H. Knoll Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 The Pros and Cons of Studying Plant Diversity Changes. . . . . . 6 The Concept of Species. . . . . . . . . . . . . . . . . . . . . . . . . 11 . . . . . . . . . Choice of Taxonomic Rank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 . Biasing Factors in the Fossil Record. . . . . . . . . . . . . . . . . . . 13 . . . . . . The "Human Factor" . . . . . . . . . . . . . . . . . . . . . . . . . 14 . . . . . . . . . . Biological Biases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Geological Biases. . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 . . . . . . . . . . . Sources, Methods, and Analyses of Apparent Diversities. . . . . . . . 26 Statistical Computations. . . . . . . . . . . . . . . . . . . . . . . . 27 . . . . . . . . . Analysis of the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 . . . . . . . . Geological Explanation of Residuals. . . . . . . . . . . . . . . . . . . 34 . . . . . . Biological Explanation of Residuals . . . . . . . . . . . . . . . . . . . . . . . . . 38 The Origin and Early Evolution of the Land Plants . . . . . . . . . . 39 The Evolution of Heterospory and the Seed Habit . . . . . . . . . . . 42 _ The Angiosperms. . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 . . . . . . . . . . . Large-Scale Biological Interpretations . . . . . . . . . . . . . . . . . 62 . . . . . .