The settlers included in this book are the animals and plants that have either been brought over intentionally or have infiltrated into this country. There are some fascinating and intriguing details about the attempts to import -- or conversely to exterminate -- the various species, and the ways that they established themselves. From this vantage point, the author manages to dramatize some important points about why and how different forms of life may or may not adapt to a new environment, how they may change as a result, and how the introduction of one organism can affect the others. The final chapter indicates some of the problems to be considered in concerted efforts to conserve or to do away with any of these types. By the author of Wanted: Amateur Scientists and Quacko and the Elps, this is a very readable presentation of some unusual material.