This quote is provided to indicate that the most significant previous major publication on the theme of this book is the volume recording the activities of the 1964 symposium (Baker and Stebbins, 1965). The volume from this conference contains many leads. Many are today not fully developed. A summary of many of the questions posed appears in the symposium's Introduction (Waddington, 1965) and the Summary (Mayr, 1965). Before reopening some of these issues, however, it is important to consider briefly what is meant by the term colonizing species, not as an absolute definition, but to set the scene for the chapters that follow. This is simply because the borderline between a colonizing species and a noncolonist is arbitrary and defined mainly by accepted usage.

At one extreme, every species is a colonist, since otherwise it would not occupy any territory. This extremely broad definition is, however, of little practical use. Often in the literature, considerations are restricted to species invading disturbed and newly created habitats following humans. With increasing urbanization and the destruction of natural habitats, such species are increasing in importance year by year. Among these species, many insects, certain rodents, and weedy plants are notably important (Baker and Stebbins, 1965). Among the insects, some of the widespread species of the dipteran genus Drosophila are important as colonizing species and have been extensively studied.