For some years one of my principal interests has been to study, in collaboration with my colleagues, E. M. Newbold, W. W. C. Topley and J. Wilson, the epidemiological phenomena observable in communities of mice exposed to risk of infection. In this work we have often found it convenient to summarise part of our results under the form of what we call a Life Table. We treat the entrants to our communities as live-births and determine the rate of mortality prevalent in the community over a certain epoch for each day of life, in the familiar life-table form. The result has a formal resemblance to a life table, but, from the nature of the case, it can have little or no bearing upon the course of events in a normal community. Naturally, however, one desired to set up some normal standard of mortality for the animal species used and, having obtained some scanty data, one was led to speculate further upon the biology of the so-called “laws” which have been from time to time proposed to describe the course of mortality in man. In this paper, I have brought together the imperfect results of such study as I have been able to make. Their practical value, from the point of view of the description of human mortality, is, it need hardly be said, negligible, while as a contribution to the history of “laws” of mortality what is omitted is perhaps as important as what is discussed. I have, however, felt justified in printing this essay in the hope that it might be accepted as a tribute in piam memoriam of my friend and colleague John Brownlee. The title of one of Brownlee's papers—“The Biology of a Life Table”—was the inspiration of much of his life-work. He approached the problem with an erudition, both biological and mathematical, to which I have no pretensions and, had his power of exposition been equal to his natural sagacity and learning, there would have been small need of any other writer.