Haeckel’s facile method of constructing genealogical trees, which ignores difficulties and discrepant facts, has met with much criticism and ridicule even among Darwinians. The “orator of Berlin,” Du Bois-Reymond, declared that if he must read romances he would prefer to read them in some other form than that of genealogical trees. But they have at least the merit that they give a vivid impression of what is most plausible and attractive in the idea of descent, and moreover they have helped towards orientation in the discussion. Nor can we ignore the very marked taxonomic and architectonic talent which their construction displays. Weismann’s Evolutionist Position. The most characteristic representative, however, of the modern school of unified and purified Darwinism is not Haeckel, but the Freiburg zoologist, Weismann. Through a long series of writings he has carried on the conflict against heterodox, and especially Lamarckian theories of evolution, and has developed his theories of heredity and the causes of variation, of the non-transmissibility of acquired characters, and the all-sufficiency of natural selection. In his latest great work, in two volumes, “Lectures on the Theory of Descent,”(11) he has definitely summed up and systematised his views. These will interest us when we come to inquire into the problem of the factors operative in evolution. For the moment we are only concerned with his attitude to the Theory of Descent as such. It is precisely the same as Haeckel’s, although he is opposed to Haeckel in regard to the strictly Darwinian standpoint. The Theory of Descent has conquered, and it may be said with assurance, for ever. That is the firm conviction on which the whole work is based, and it is really rather treated as a self-evident axiom than as a statement to be proved. Weismann takes little trouble to prove it. All the well-known, usually very clear proofs from palæontology, comparative anatomy, &c., which we are accustomed to meet with in evolutionist books are wanting here, the genealogical trees of the Equidæ, with the gradually diminishing number of toes and the varying teeth, of _Planorbis multiformis_, of the ammonites, the graduated series of stages exhibited by individual organs, for instance, from the ganglion merely sensitive to light up to the intricate eye, or from the rayed skeleton of the paired fins in fishes up to the five-fingered hands and feet of the higher vertebrates, &c. These are only briefly touched upon in the terse “Introduction,” and the whole of the comprehensive work is then directed to showing what factors can have been operative, and to proving that they must have been “Darwinian” (selection in the struggle for existence), and not Lamarckian or any other. This is shown in regard to the coloration of animals, the phenomena of mimicry, the protective arrangements of plants, the development of instinct in animals, and the origin of flowers.