The phrase “ontogeny recapitulates phylogeny,” sometimes called simply the theory of recapitulation, appears in most introductory biology texts, as an idea with a long heritage. It is sometimes called the “biogenetic law” or “embryological parallelism.” But what a modern zoologist means by that phrase is far different from the concept it originally signified.
Definitions of ontogeny and phylogeny
Ontogeny is the origin, growth, and development of an organism. It consists of the changes that occur in an individual from the beginning of its life (fertilization in sexually reproducing species) through adulthood and senescence. It refers both to increasing size and to changing shape of the organism. In the context of recapitulation theory, it is primarily concerned with the change in shape of the embryo.
Phylogeny is the evolutionary history of a species. Unlike ontogeny, phylogeny applies to entire populations. It consists of the changes that occurred in the body form of the species' ancestors from the origin of life to the present and in the relationships among different species. These relationships can be represented in tree form, sometimes called a phylogeny tree or cladogram; the phylogeny tree for all life on earth is called the Tree of Life.
Although the two concepts, ontogeny and phylogeny, are different, the phrase “ontogeny recapitulates phylogeny” posits a relationship between them. Scientists' understanding of the nature of that relationship has changed since the origin of the idea.
Ernst Haeckel’s model
German naturalist Ernst Haeckel (1834-1919) coined the phrase “ontogeny recapitulates phylogeny” in 1866. Although this was several years after Charles Darwin’s On the Origin of Species, Haeckel was not an adherent of Darwin’s evolution through natural selection; he believed in Lamarckian evolution. Lamarckism was an early theory in which adaptations were thought to result from a rather mysterious process involving use or disuse of organs; then these acquired characteristics were passed on to offspring.
In Lamarckian evolution, as in the modern theory of evolution that began with Darwin’s ideas, all species thus have a phylogeny, or evolutionary history. In fact, Haeckel is the one who coined the term phylogeny. Haeckel proposed that every individual organism’s ontogeny directly reflected its phylogeny. According to this idea, a chick embryo early in its development looks exactly like a fish, complete with fins, gills, and a fishy tail. Next it becomes exactly like a reptile, and finally becomes recognizable as a chick. Likewise, a human embryo also begins as a fish, then a reptile, an early mammal, and finally a human.
Haeckel’s ideas about recapitulation have been discredited, but they influenced thinking in other disciplines such as psychology.