The attention of those interested in mental diseases has lately been directed towards theories of the causes and origin of these diseases. In two recent works, widely differing in scientific scope and aim, though respectively instructive and powerful, theories of the genesis of melancholia and mania based upon the latest psychological and physiological knowledge have been advanced. To my mind, these theories, though containing much statement that is incontrovertible, stop short of conclusive, satisfying argument. At the same time, I was much surprised to read the following remarkable passage in a review of Dr. Mercier's book in the pages of the last number of this Journal:— “It is well to have a devout follower of Hughlings Jackson and Herbert Spencer, or, perhaps, one ought to have said a follower of Darwin and of evolution, giving his views upon mental order and disorder…. We ourselves are still inclined to believe in the unknowable.” Is the clinical and scientific work of the past few years, based upon the writings of, to put them in their proper order, Herbert Spencer, Darwin, and Hughlings Jackson, to be undone and disregarded, and are we to calmly fold our hands and wait with the reviewer for a new revelation? It will be strange, indeed, if any future theory of “mental order or disorder” can exist without having to reckon with the theories of Herbert Spencer and of evolution.