The names of great Catholic men of science, laymen like Pasteur and Mueller, or ecclesiastics like Stensen and Mendel, are familiar to all educated persons. But even educated persons, or at least a great majority of them, are quite ignorant of the goodly band of workers in science who were devout children of the Church. Nothing perhaps more fully exemplifies this than the history of the controversy respecting the subject whose name is set down as the title of this paper. For centuries a controversy raged at intervals around the question of spontaneous generation. Did living things originate, not merely in the past but every day, from non-living matter? When we consider such things as the once mysterious appearance of maggots in meat it is not wonderful that in the days before the microscope the answer was in the affirmative. To-day the question may be considered almost closed. True, the negative proposition cannot be proved, hence it is impossible to say that spontaneous generation does not take place. However, the scientific world is at one in the belief that so far all attempts to prove it have failed utterly. St. Thomas Aquinas had a celebrated and sometimes misunderstood controversy with Avicenna, a very famous Arabian philosopher. It was a philosophical, but not strictly scientific, controversy, for both persons accepted or assumed the existence of spontaneous generation. Avicenna claimed that it took place by the powers of Nature alone, whilst St. Thomas adopted the attitude which we should adopt to-day, were spontaneous generation shown to be a fact, namely, that if Nature possessed this power, it was because the Creator had willed it so. We come to close quarters with the question itself in 1668, when Francesco Redi (1626-1697) published his book on the generation of insects and showed that meat protected from flies by wire gauze or parchment did not develop maggots, whilst meat left unprotected did. From this and from other experiments he was led to formulate the theory that in all cases of apparent production of life from dead matter the real explanation was that living germs from outside had been introduced into it. For a long time this view held the field. Redi was, as his name indicates, an Italian, an inhabitant of Aretino, a poet as well as a physician and scientific worker. He was physician to two of the Grand Dukes of Tuscany and an academician of the celebrated _Accademia della Crusca_. Those works which I have been able to consult on the subject say nothing about his religion, but there can scarcely be any doubt that he was a Catholic. At any rate there is no doubt whatever as to the other persons now to be mentioned in connection with the controversy, which again became active about a century after Redi had published his book. The antagonists on this occasion were both of them Catholic priests, and both of them deserve some brief notice.