Birth and death are notoriously the only two certainties in life. Men have always desired immortality, have been willing to pay any price in order to procure it, and have always been denied it. They have not ceased to ask questions about it. Since the origins of scientific biology, two recurrent themes have been whether all organisms are born, and whether all must die; and the notion that there is a deep connection between these two invariants has fuelled speculation from the earliest times down to the present day. Spontaneous generation was gradually excised from the scientific curriculum, or at any rate pushed far back into the geological fog, but the possibility of immortality has not been so easy to dispose of. All our familiar animals and plants must surely die, as we must ourselves; even when protected against all the usual rigours of life, the approach of death is eventually signalled by a progressive deterioration, an irreversible process of senescence. It was not obvious that the same should be true of the new world of minute creatures revealed by the invention of the microscope towards the end of the seventeenth century. Protozoans like the Amoeba and Paramecium familiar from introductory biology classes grow, divide into two apparently identical cells, grow and divide again. They may readily be killed, of course, by almost any sort of minor physical or chemical insult, but in principle, given favourable conditions and protected from shocks, perhaps the cycle of growth and division would continue indefinitely, and thereby prove that natural death was not after all inevitable.