This portion of the subject has, during the past two or three years, deepened and strengthened in interest; our knowledge of it has largely increased; and the recognition of its vital importance has called forth, both in Europe and America, a host of eager and capable workers. It is scarcely too much to say that its wide and far-reaching issues are probably the most important to mankind of any that have been studied in recent times. Foremost amongst the workers in this department may be recalled to you the well-known names of Koch and Pasteur, abroad; and of Lister, Watson-Cheyne, and Crookshank, in this country. But the names of other eminent investigators will almost necessarily occur to your minds. It has long been known what potent factors were microscopic Germs in producing changes in the constitution of decaying matter; and how they were, in all probability, the useful scavengers of nature, definitely resolving into their constituent elements failing and dying organic tissues. But it has become more and more a matter of knowledge, that by their parasitic habits and their power of invading and living upon and within other living tissues, they are also the sources of many so-called diseases of both vegetables and animals. With regard to vegetable parasitic growth I will not now detain you. The blights, and the mildews, and the ergots, as well as minuter forms, such as algæ and micrococci, are well known; and no doubt much more is still to be learned from their further investigation; whilst by analogy it seems highly probable that the circulating fluids of higher members of the vegetable kingdom may be found to be invaded by parasitic beings in the same way as their animal compeers. But it is to Germ life in animal bodies that I wish now specially to allude. I have said that our knowledge in this direction has, even in the last two or three years, made enormous strides, and it is now almost a matter of certainty that all contagious or infectious disorders, as well as many others, are but the expression of the fact that minute living bodies have made a resting-place for themselves in or upon other living tissues; and that the development of the phenomena of these morbid states is but an indication of their presence and reproductive activity—either as cause, or as an accompaniment of these disease manifestations. And this applies not only to the human species, but to lower classes of animals, between whom and man many of these diseases are interchangeable. Such knowledge necessarily invests the life history of these minute Germs with intense interest, seeing that it is probably one step, and that a long one, towards the discovery of the means of prevention, if not also of cure, of many of our most fatal and dreaded diseases.