This collection of family letters was written over a period of fifty years by George G. Simpson who was an inveterate letter writer, especially to his older sister Martha (1908-1984) and his parents. Martha recognized very early her brother's special intellectual gifts and as a measure of her affection and admiration managed to save 156 of his letters, despite her own many moves about the United States and Europe. She eventually entrusted them to the archives of the American Philosophical Society in Philadelphia. Another 54 World War II letters to his parents and sisters were discovered among Simpson's personal papers after his death; these are also included in the volume.

I learned of the existence of the letters during conversations with Martha Simpson Eastlake in Tucson, Arizona, when doing my initial background research on Simpson's professional life and scientific work. As soon as practicable I went to Philadelphia and spent several days reading the letters. I was immediately convinced that they would interest a much wider audience, so I had them photocopied and sent to me in California.

I then transcribed all the letters, all but a few of them handwritten. Although Simpson's handwriting was more legible than most people's, when in a whimsical mood he would do mirror-writing, or write in tightly coiled spirals around and around on the page, or in French, Spanish, or doggerel German. Some sentences are even in his own made-up code. Because of the informality of the letters, Simpson was not always careful or consistent about his punctuation. His French and Spanish, too, are often colloquial or ungrammatical. I have let the idiosyncrasies remain, except in instances of obvious errors that would mislead readers.

The letters are arranged in chronological order. In most cases Simpson dated them, but for those undated I have relied on internal evidence for their chronological position. The letters are also divided into somewhat arbitrary, but logical, sections. For each section I have written a short introduction to provide the biographical context for better understanding. Brief footnotes are also included to identify or clarify persons, places, and events that might be unknown to the general reader.

I thank the American Philosophical Society for granting permission to publish the letters and the volume is dedicated to two older sisters, Martha Simpson Eastlake and Joanne Laporte Sheridan, whose affectionate support could always be counted on by their younger brothers.