In the article “Mortality”, in the Penny Cyclopæia, the late Professor De Morgan gave the following description of the law of mortality propounded by Gompertz in the paper presented to the Royal Society in 1825. “As this ingenious paper”, says De Morgan, “contains a deduction from a principle of high “probability, and terminates in a conclusion which accords in a “great degree with observed facts, it must always be considered “a very remarkable page in the history of the enquiry before “us…. There is in the human constitution a power of “resisting the effects of disease which increases from birth up to “a certain age, and diminishes from that time forwards.… “Mr. Gompertz assumes that the power to oppose destruction “loses equal proportions in equal times, &c, &c.” Gompertz's theory of mortality, then, is based upon the supposed physiological fact that the living human organism is endowed with a certain recuperative power, becoming (after a few years from birth) ever less and less efficient with the lapse of time, which he terms “the power to oppose destruction”, but which, for brevity, I will call “vital force”, the truth of which supposition is evidently a question for common observation.