This book began when, out of curiosity, I glanced at the contents of the first issue of the Quarterly Review of Biology and saw an article with the irresistible title of ‘Eleven thousand generations of Paramecium’. Its author, L. L. Woodruff, was describing how he had maintained a strain of Paramecium aurelia in isolate culture for 19 years, with no sign of any decrease in vitality. My first thought was that this would provide an excellent illustration of the futility of research unguided by adequate theoretical understanding. On second thoughts, there might be more to the problem than met the eye; and little by little I was drawn into reading most of a huge literature, the bulk of it written between 1890 and 1920, generated by the speculation that sex has some ill-defined rejuvenating property that made it indispensable to the indefinite prolongation of a line of protozoans. Although most geneticists and protozoologists (I am neither) are aware of this literature, it has long since faded from the foreground, and the musty volumes in which it lies entombed are nowadays little read. I suspect that most people have been put off by the almost mystical properties ascribed to sex by many of these early workers, at a time when Mendelism had only just been rediscovered and the concept of the genotype was still novel. The attempts to ‘revivify’ failing cultures with extract of pancreas, for example, read rather strangely today. A large part of this book is simply historical, therefore, born of my fascination with the attempts of these early geneticists to grapple with ideas of sexuality and immortality, and of my own attempts to understand their results in modern terms.