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.