Embryology 143 
I had endeavoured to show in 1874, in the first chapter of my 
Anthropogenic’, 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®.” 
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 Darwin? (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*. 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. 
* Eng, transl. ; Facts and Arguments for Darwin, London, 1869. 
* 3 vols., Berlin, 1894—96. 
