LECTUEE XXVII 

 THE BIOGENETIC LAW 



Fritz Muller's ideas — Development of the Crustaceans — Of the Daphnidae — Of 

 Sacculina — Of parasitic Copepods — Larvse of the higher Crustaceans — Change of 

 phyletic stages in Ontogeny — Haeckel's Fundamental Biogenetic Law — Palingenesis and 

 Coenogenesis — Variation of phyletic forms by interpolation in a lengthened Ontogeny 

 — Justification of deductions from Ontogeny to Phylogeny — Wilrtemberger's series 

 of Ammonites — Phylogeny of the markings in the caterpillars of the Sphingidiie — 

 Condensation of Phylogeny in Ontogeny — Example from the Crustaceans — Disappear- 

 ance of useless parts — ^The variation of homologous parts, according to Emery — Germ- 

 plasmic correlations — Harmony vrith the theory of determinants — Multiplication of the 

 determinants in the course of the phylogeny. 



What I propose to discuss in this lecture should have been 

 considered at an earlier stage, if we had pledged ourselves to adhere 

 strictly to the historical sequence of scientific discovery, for the 

 phenomena which we are about to deal with attained recognition 

 shortly after the revival of the evolution idea, and indeed they 

 formed the first important discovery which was made on the basis 

 of the Darwinian Doctrine of Descent. I have introduced them at 

 this stage because they have to do with phenomena of inheritance 

 and modifications of these, the understanding of which — in as far 

 as we can as yet speak of understanding at all — is only possible 

 on the basis of a theory of inheritance. Therefore, in order to 

 examine these phenomena and their causes, it was necessary first 

 to submit a theory of heredity, as I have done in the germ-plasm 

 theory. We have to treat of the connexion between the development 

 of many-celled individuals and the evolution of the species, between 

 germinal history and racial history, or, as we say with Haeckel, 

 between ontogeny and phylogeny. 



Long before Darwin's day individual naturalists had observed 

 that certain stages in the development of the higher vertebrates, such 

 as birds and mammals, showed a likeness to fishes, and they had spoken 

 of a fish-like stage of the bird-embryo. The ' Natural Philosophers ' 

 of the beginning of the nineteenth century, Oken, Treviranus, Meckel, 

 and others, had, on the basis of the transmutation theory of the time, 

 gone much further, and had professed to recognize in the embryonic 

 history of Man, for example, a repetition of the difierent animal 



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