Embryology 143 



I had endeavoured to show in 1874, in the first chapter of my 

 Anthropogenic 1 , that this fundamental law of organic evolution 

 holds good generally, and that there is everywhere a direct causal 

 connection between ontogeny and phylogeny. "Phylogenesis is 

 the mechanical cause of ontogenesis " ; in other words, " The 

 evolution of the stem or race is — in accordance with the laws of 

 heredity and adaptation — the real cause of all the changes that 

 appear, in a condensed form, in the development of the individual 

 organism from the ovum, in either the embryo or the larva." 



It is now fifty years since Charles Darwin pointed out, in the 

 thirteenth chapter of his epoch-making Origin of Species, the 

 fundamental importance of embryology in connection with his theory 

 of descent : 



"The leading facts in embryology, which are second to none in 

 importance, are explained on the principle of variations in the many 

 descendants from some one ancient progenitor, having appeared at 

 a not very early period of life, and having been inherited at a 

 corresponding period 2 ." 



He then shows that the striking resemblance of the embryos and 

 larvae of closely related animals, which in the mature stage belong to 

 widely different species and genera, can only be explained by their 

 descent from a common progenitor. Fritz Miiller made a closer 

 study of these important phenomena in the instructive instance of 

 the Crustacean larva, as given in his able work Fiir Danvin 3 (1864). 

 I then, in 1872, extended the range so as to include all animals (with 

 the exception of the unicellular Protozoa) and showed, by means of 

 the theory of the Gastraea, that all multicellular, tissue-forming 

 animals — all the Metazoa — develop in essentially the same way from 

 the primary germ-layers. I conceived the embryonic form, in which 

 the whole structure consists of only two layers of cells, and is 

 known as the gastrula, to be the ontogenetic recapitulation, main- 

 tained by tenacious heredity, of a primitive common progenitor of 

 all the Metazoa, the Gastraea. At a later date (1895) Monticelli 

 discovered that this conjectural ancestral form is still preserved in 

 certain primitive Coelenterata — Pemmatodiscus, Kunstleria, and the 

 nearly-related Orthonectida. 



The general application of the biogenetic law to all classes 

 of animals and plants has been proved in my Systematische 

 Phylogenie 4 . It has, however, been frequently challenged, both by 

 botanists and zoologists, chiefly owing to the fact that many have 

 failed to distinguish its two essential elements, palingenesis and 



1 Eng. transl.; The Evolution of Man, 2 vols., London, 1879 and 1905. 



2 Origin of Species (6th edit.), p. 396. 



3 Eng. transl. ; Facts and Arguments for Darwin, London, 1869. 

 * 3 vols., Berlin, 1894—96. 



