Several years ago Lord Rosebery founded, in the University of Edinburgh, a lectureship on “The Philosophy of Natural History,” and I was invited by the Senatus to deliver the lectures. This invitation I accepted, and subsequently constituted the material of my lectures the foundation of another course, which was given in the Royal Institution, under the title “Before and after Darwin.” Here the course extended over three years—namely from 1888 to 1890. The lectures for 1888 were devoted to the history of biology from the earliest recorded times till the publication of the “Origin of Species” in 1859; the lectures for 1889 dealt with the theory of organic evolution up to the date of Mr. Darwin’s death, in 1882; while those of the third year discussed the further developments of this theory from that date till the close of the course in 1890.

            It is from these two courses—which resembled each other in comprising between thirty and forty lectures, but differed largely in other respects—that the present treatise has grown. Seeing, however, that it has Grown much beyond the bulk of the original lectures, I have thought it desirable to publish the whole in the form of three separate works. Of these the first—or that which deals with the purely historical side of biological science—may be allowed to stand over for an indefinite time. The second is the one which is now brought out and which, as its sub-title signifies, is devoted to the general theory of organic evolution as this was left by the stupendous labours of darwin. as soon as the translations shall have been completed, the third portion will follow (probably in the autumn season), under the sub-title, “post-darwinian questions.”