EMBRYOLOGY AND EVOLUTION
embryology,” recognized that this view was not wholly correct, and he modified it as follows: “An embryo never resembles an adult animal and is only to be compared with the embryos of other animals. The more different two animal forms are in their end stages the farther back in their development must one go in order to find agreement between them.” This has often been called “von Baer's Law.” Louis Agassiz, in his famous Essay on Classification (1858), pointed out the fact that there is a parallelism between embryology, palaeontology, and classification in that the stages that an animal passes through in its development from the egg resemble certain animal forms that have appeared in the past history of the earth and also certain lower forms now living.
The full significance of this parallelism was not appreciated until the revival of the doctrine of evolution under Darwin. In the fourteenth chapter of the Origin of Species Darwin discusses this parallelism and the significance of the homologies of embryos, and he closes his discussion of embryology with these carefully guarded words: “Embryology rises greatly in interest when we look at the embryo as a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class.”
It was Ernst Haeckel, in his Generelle Morphologie (1866) and in many later books, who announced that “Ontogeny is a short recapitulation of Phylogeny” — that is, the successive embryonic stages in the development of an animal correspond to the successive adult stages of the phylum to which it belongs. This is Haeckel's Fundamental Law of Biogeny (“Biogenetisches Grundgesetzt”), which is more frequently called the theory of embryonic recapitulation.
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