We _must_ assume determinants, and consequently the germ-plasm must have room for these; the variations of species can only be explained through variations of the germ-plasm, for these alone give rise to hereditary variation. It is upon this foundation that my germinal selection is built up; whether I have in the main reached the truth the future will show: but that I have not exhausted this new domain, but only opened it up, I am very well aware. LECTURE XXVII THE BIOGENETIC LAW Fritz Müller's ideas--Development of the Crustaceans--Of the Daphnidæ--Of Sacculina--Of parasitic Copepods--Larvæ of the higher Crustaceans--Change of phyletic stages in Ontogeny--Haeckel's _Fundamental Biogenetic Law_--Palingenesis and Cœnogenesis--Variation of phyletic forms by interpolation in a lengthened Ontogeny--Justification of deductions from Ontogeny to Phylogeny--Würtemberger's series of Ammonites--Phylogeny of the markings in the caterpillars of the Sphingidæ--Condensation of Phylogeny in Ontogeny--Example from the Crustaceans--Disappearance of useless parts--The variation of homologous parts, according to Emery--Germ-plasmic correlations--Harmony with 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.