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 [pg 104] 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.

In reality Weismann only adduces one strict proof, and even that is only laying special stress on what is well known in comparative embryology; namely, the possibility of “predicting” on the basis of the theory of descent, as Leverrier “predicted” Neptune. For instance, in the lower vertebrates from amphibians upwards there is an os centrale in the skeleton of wrist, but there is none in man. Now if man be descended [pg 105] from lower vertebrates, and if the fundamental biogenetic law be true (that every form of life recapitulates in its own development, especially in its embryonic development, the evolution of its race, though with abbreviations and condensations), it may be predicted that the os centrale is to be found in the early embryonic stages of man. And Rosenberg found it. In the same way the “gill-clefts” of the fish-like ancestors have long since been discovered in the embryo of the higher vertebrates and of man. Weismann himself “predicted” that the markings of the youngest stage of the caterpillars of the SphingidÆ (hawk-moths) would be found to be not oblique but longitudinal stripes, and ten years later a fortunate observation verified the prediction. Because of the abundance of evidential facts Weismann does not go into any detailed proof of evolution. “One can hardly take up any work, large or small, on the finer or more general structural relations, or on the development of any animal, without finding in it proofs for the evolution theory.”

But assured as the doctrine of descent appears,12 and certain as it is that it has not only maintained its hold since Darwin's day, but has strengthened it and has gained adherents, this foundation of Darwinism is [pg 106] nevertheless not the unanimous and inevitable conclusion of all scientific men in the sense and to the extent that the utterances of Weismann and others would lead us to suppose. Apart from all apologetic attempts either in religious, ethical, or Æsthetic interests, apart, too, from the superior standpoint of the philosophers, who have not, so to speak, taken the theory very seriously, but regard it as a provisional theory, as a more or less necessary and useful method of grouping our ideas in regard to the organic world, there are even among the biologists themselves some who, indifferent towards religious or philosophical or naturalistic dogma, hold strictly to fact, and renounce with nonchalance any pretensions at completeness of knowledge if the data do not admit of it, and on these grounds hold themselves aloof from evolutionist generalisation. From among these come the counsels of “caution,” admissions that the theory is a scientific hypothesis and a guide to research, but not knowledge, and confessions that the Theory of Descent as a whole is verifiable rather as a general impression than in detail.