I. Haeckel and His Opponents

Weighty evidence against such attempts to introduce ideal formative forces into the developmental history of the individual, is afforded by the achievements of those investigators who have really explained the forms of higher living creatures on the assumption that these forms are the hereditary repetition of innumerable historical changes in the history of the species, which have occurred during long ages. A striking example in this respect is the “vertebral theory of the skull-bones,” already dimly anticipated by Goethe and Oken, but first set in the right light by Carl Gegenbauer on the basis of the theory of descent. He demonstrated that the skull of the higher vertebrates, and also that of man, has arisen from the gradual transformation of a “root-skull” whose form is still preserved by the “root-fishes,” or primordial gastrea, in the formation of the head. Supported by such results, Gegenbauer therefore remarks rightly: “The descent theory will likewise find a touch-stone in comparative anatomy. Hitherto there existed no observation in comparative anatomy which contradicts it; all observations rather lead us towards it. Thus that theory will receive back from comparative anatomy what it gave to its method: clearness and certainty” ( cp. the Introduction to Gegenbauer's Vergleichende Anatomie). The descent theory has directed science to seek for the real causes of the individual development of each organism in its ancestry; and natural science on this road replaces the ideal laws of development which might be supposed to superpose themselves on organic matter, by the actual facts of the ancestral history, which continue to operate in the individual creature as formative forces.