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 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 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.