This preface is addressed to the reader who wishes to inquire into the prevailing concepts, hypotheses and theories about development of sensory systems and wants to know how they are exemplified in the following chapters. I believe that science is hypothesis and theory and that the growth and evolution of any branch of science can be measured by the degree to which its theories have been reified. By that standard, one must conc1ude that developmental neuro biologie is in its infancy. The rapid accumulation of observations which has occurred in this branch of science in the past century leads to progress only to the extent that the facts validate or falsify hypotheses. The following chapters show that we have a plethora of facts but a dearth of hypotheses. Another index of the maturity of any branch of science is its level of historical self-awareness. Because the history of any branch of science is essentially the history of ideas and of the rise and fall of theories, the level of historical awareness is related to the extent to which reification of its hypothetical constructs has advanced. It is largely because few theories of development of sensory systems, or indeed, of developmental neurobiology, have progressed far in the process of reification that the his tory of developmental neurobiology remains unwritten. The subject of this volume is hardly mentioned in the many books devoted to the history of related disciplines."

Migration, broadly defined as directional movement to take advantage of spatially distributed resources, is a dramatic behaviour and an important component of many life histories that can contribute to the fundamental structuring of ecosystems. In recent years, our understanding of migration has advanced radically with respect to both new data and conceptual understanding. It is now almost twenty years since publication of the first edition, and an authoritative and up-to-date sequel that provides a taxonomically comprehensive overview of the latest research is therefore timely. The emphasis throughout this advanced textbook is on the definition and description of migratory behaviour, its ecological outcomes for individuals, populations, and communities, and how these outcomes lead to natural selection acting on the behaviour to cause its evolution. It takes a truly integrative approach, showing how comparisons across a diversity of organisms and biological disciplines can illuminate migratory life cycles, their evolution, and the relation of migration to other movements. Migration: The Biology of Life on the Move focuses on migration as a behavioural phenomenon with important ecological consequences for organisms as diverse as aphids, butterflies, birds and whales. It is suitable for senior undergraduate and graduate level students taking courses in behaviour, spatial ecology, 'movement ecology', and conservation. It will also be of interest and use to a broader audience of professional ecologists and behaviourists seeking an authoritative overview of this rapidly expanding field.

This volume is the result of a symposium entitled "Variation in Life Histories: Genetics and Evolutionary Processes" sponsored by the Program in Evolutionary Ecology and Behavior ofthe University of Iowa and held in Iowa City on October 13 and 14, 1980. Prompted by arecent upsurge of interest in the evolution of life histories, we chose this topic because of the obvious association between life history traits and Darwinian fit- ness. If such an association were to be fruitfully investigated, it would require the eloser cooperation of population and evolutionary ecologists and quantitative and population geneticists. To encourage such an association, our symposium had four major aims: first, to facilitate intellectual exchange across disciplines among an array of biologists studying life histories; second, to encourage exploration of genetic variance and covari- ance for life history traits; third, to consider the ecological background for genetic vari- ability; and finally, to facilitate a comparative overview both within and among species. Obviously such broad aims cannot be met totally in a single volume, but we think we have succeeded reasonably wen in providing a representative and nourishing intel- lectual feast. We see this book as a stimulus to the coordination of future efforts in an important and expanding area of inquiry. We have divided the book into six sections.

Migration, broadly defined as directional movement to take advantage of spatially distributed resources, is a dramatic behaviour and an important component of many life histories that can contribute to the fundamental structuring of ecosystems. In recent years, our understanding of migration has advanced radically with respect to both new data and conceptual understanding. It is now almost twenty years since publication of the first edition, and an authoritative and up-to-date sequel that provides a taxonomically comprehensive overview of the latest research is therefore timely. The emphasis throughout this advanced textbook is on the definition and description of migratory behaviour, its ecological outcomes for individuals, populations, and communities, and how these outcomes lead to natural selection acting on the behaviour to cause its evolution. It takes a truly integrative approach, showing how comparisons across a diversity of organisms and biological disciplines can illuminate migratory life cycles, their evolution, and the relation of migration to other movements. Migration: The Biology of Life on the Move focuses on migration as a behavioural phenomenon with important ecological consequences for organisms as diverse as aphids, butterflies, birds and whales. It is suitable for senior undergraduate and graduate level students taking courses in behaviour, spatial ecology, 'movement ecology', and conservation. It will also be of interest and use to a broader audience of professional ecologists and behaviourists seeking an authoritative overview of this rapidly expanding field.