I am aware that what I may say on this head is likely to prove more interesting to future students of the literature of descent than to my immediate public, but any book that desires to see out a literary three-score years and ten must offer something to future generations as well as to its own. It is a condition of its survival that it shall do this, and herein lies one of the author’s chief difficulties. If books only lived as long as men and women, we should know better how to grow them; as matters stand, however, the author lives for one or two generations, whom he comes in the end to understand fairly well, while the book, if reasonable pains have been taken with it, should live more or less usefully for a dozen. About the greater number of these generations the author is in the dark; but come what may, some of them are sure to have arrived at conclusions diametrically opposed to our own upon every subject connected with art, science, philosophy, and religion; it is plain, therefore, that if posterity is to be pleased, it can only be at the cost of repelling some present readers. Unwilling as I am to do this, I still hold it the lesser of two evils; I will be as brief, however, as the interests of the opinions I am supporting will allow. In “Life and Habit” I contended that heredity was a mode of memory. I endeavoured to show that all hereditary traits, whether of mind or body, are inherited in virtue of, and as a manifestation of, the same power whereby we are able to remember intelligently what we did half an hour, yesterday, or a twelvemonth since, and this in no figurative but in a perfectly real sense. If life be compared to an equation of a hundred unknown quantities, I followed Professor Hering of Prague in reducing it to one of ninety-nine only, by showing two of the supposed unknown quantities to be so closely allied that they should count as one. I maintained that instinct was inherited memory, and this without admitting more exceptions and qualifying clauses than arise, as it were, by way of harmonics from every proposition, and must be neglected if thought and language are to be possible.