Development

The eighteenth-century debates about preformism and epigenesis brought development into the spotlight of biological investigations. Ordering the known diversity of life, increasing by the day as a result of European voyages of exploration, was another major concern. For many, especially the Romantic scientists at the turn of the nineteenth century, these two areas of natural history were connected. Did the diversity of nature not arise in the course of development from similar structures? Are the creative principles in nature not the same as in the arts? Pondering these questions on a trip to Italy, the poet-philosopher-scientist Johann Wolfgang von Goethe discovered the principle of metamorphosis and established the foundations of morphology. Specifically, Goethe realized that all the diverse structures of flowering plants are transformations of one basic morphological form, the leaf. Understanding these principles of transformation, or metamorphosis, then allows the scientist or the artist to recreate all existing organic forms, as well as those that could exist but have not yet been realized. This morphological building plan (Bauplan) is intrinsically dynamic and developmental; it is a principle that unfolds itself in nature small and large, in the individual and the cosmos. Morphogenesis focused, for Goethe and others, on the emergence of form within a context of change.

Ideas about transformation were soon applied to species as well as individuals. In 1809 Jean-Baptiste de Lamarck published his theory of evolution, which gave development the additional meaning of the transmutation of species. For Lamarck the transmutation of species was driven by an intrinsic drive toward perfection. In this "escalator theory" of evolution, primitive forms, created spontaneously, pass through increasingly complex stages in the course of subsequent generations. The essence of nature is thus transformation, both in the course of individual development and in the generation of the diversity of life. Lamarck's theory was readily attacked, especially by his colleague George Cuvier, the founder of comparative anatomy. Cuvier had established the most sophisticated classification system of animals of his time, based on the recognition of four distinct types of animals and a strict hierarchy of systematic categories within each of these embranchements. Within this system species were considered immutable, and their relationships were defined by the degree of similarity between them.

Development was one way of explaining this similarity among species. All organisms begin their life as fertilized eggs—Karl Ernst von Baer would discover the mammalian egg in 1827—and the early stages of development also resemble each other more closely than later stages. Summarizing these observations, Johann Friedrich Meckel proposed in 1811 that the embryological stages of advanced organisms represent the adult stages of more primitive organisms. This was the first formulation of the principle of recapitulation, in which development became the causal explanation for the similarity as well as the differences between species. The evolutionary implications were obvious. Defending the clear separation of different systematic groups, Karl Ernst von Baer summarized his opposition to the principle of recapitulation in his developmental laws. He stated that no adult organism is like any embryo of another organism, that each developmental trajectory is unique, but that in each developmental sequence the more general features of the organismal structure appear earlier in development, which explains the close similarities between the early embryos of different species.

Von Baer's authority carried the day, but only briefly. In his theory of evolution as descent with modification, Charles Darwin also relied on embryological evidence, especially when he needed a mechanism that would explain the origin of new variations. Another consequence of the Darwinian theory was that the historical connections between species, their genealogy, immediately suggested an explanation for the similarity between them. The more closely two species are related to each other, the more similar they will be. Homologies, those structures that were considered the same in different organisms, could now be explained as being derived from a common ancestor. The only practical problem was that the genealogical relations between species were not obvious and needed to be inferred based on the similarity between them.

Studying the development of different species offered a way to escape this circularity of reasoning. Ernst Haeckel postulated that ontogeny recapitulates phylogeny, that the developmental sequence of an individual parallels the historical sequence of evolution. For Haeckel development was simultaneously a record of history and an explanation of diversity, as new structures would occur as terminal additions in the developmental process. Development also provided a way to establish homologies; those structures that were derived from the same embryological precursors (anlagen) could be considered to be homologies and used for the reconstruction of phylogenies. Haeckel's ideas, largely discredited today, were extremely influential in the second half of the nineteenth century and led to many proposals about the shape of the "tree of life."