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-sufficie
cy 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 palaeontology, comparative anatomy, &c., which we are
accustomed to meet with in evolutionist books are wanting here, the
genealogical trees of the Equidae, 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 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 Sphingidae (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 aesthetic 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.