These two branches of our science—on the one side ontogeny or embryology, and on the other phylogeny, or the science of race-evolution—are most vitally connected. The one cannot be understood without the other. It is only when the two branches fully co-operate and supplement each other that “Biogeny” (or the science of the genesis of life in the widest sense) attains to the rank of a philosophic science. The connection between them is not external and superficial, but profound, intrinsic, and causal. This is a discovery made by recent research, and it is most clearly and correctly expressed in the comprehensive law which I have called “the fundamental law of organic evolution,” or “the fundamental law of biogeny.” This general law, to which we shall find ourselves constantly recurring, and on the recognition of which depends one’s whole insight into the story of evolution, may be briefly expressed in the phrase: “The history of the fœtus is a recapitulation of the history of the race”; or, in other words, “Ontogeny is a recapitulation of phylogeny.” It may be more fully stated as follows: The series of forms through which the individual organism passes during its development from the ovum to the complete bodily structure is a brief, condensed repetition of the long series of forms which the animal ancestors of the said organism, or the ancestral forms of the species, have passed through from the earliest period of organic life down to the present day.
The causal character of the relation which connects embryology with stem-history is due to the action of heredity and adaptation. When we have rightly understood these, and recognised their great importance in the formation of organisms, we can go a step further and say: Phylogenesis is the mechanical cause of ontogenesis.[1] In other words, the development of the stem, or race, is, in accordance with the laws of heredity and adaptation, the cause of all the changes which appear in a condensed form in the evolution of the fœtus.
[1] The term “genesis,” which occurs throughout, means, of course, “birth” or origin. From this we get: Biogeny = the origin of life (_bios_); Anthropogeny = the origin of man (_anthropos_); Ontogeny = the origin of the individual (_on_); Phylogeny = the origin of the species (_phulon_); and so on. In each case the term may refer to the process itself, or to the science describing the process.—Translator.