Of Embryos and Transmutation Part I – A Genealogy of Evolution

By Robbert Striekwold

Ideas concerning species transformation and embryonic development enjoy a long and quirky history of perceived parallels and cross-pollination. In the first and introductory part of this series, I will take a look at the etymology of the word ‘evolution’ in biology. It entered the field as a term in embryology in the 18^th century, to refer to theories of preformation. In little over a century, however, it made a U-turn and came to designate the general process of species transformation.[1]

In 1744, the great Swiss anatomist Albrecht von Haller (1708-1777) used the word ‘evolution’ (meaning ‘to unroll’ or ‘to unfold’) to describe a certain 17^th century theory of embryonic development. The theory in question was that of preformationism, which posited that the form of the adult animal was already contained in the egg or sperm, and that growth and expansion were all that was needed to make it develop into a full-grown specimen. In contrast, the theory of epigenesis proposed a movement from an undifferentiated germ towards a complex, differentiated adult, guided by an organizing  force.

Haller, who had moved from a preformationist position to epigenesis, would later return to preformationism again, largely on the same grounds as the famous Dutch entomologist Jan Swammerdam (1637-1680) and many others. One of the main problems with epigenesis was that it required a goal-directed (Aristotelian) force to guide it, while preformationism needed nothing but mechanical (Newtonian) forces. In this Newtonian age it is thus hardly surprising for Haller to observe that “the theory of evolution proposed by Swammerdam and Malpighi prevails almost everywhere.”[2]

Epigenesis and Evolution

The second half of the 18^th century, however, saw an increased interest in epigenesis. This was in part due to better microscopes, which made it possible to spot embryos in early stages where they still lacked certain structures, and in part due to an increased willingness of naturalists to consider the use of non-mechanical forces in their theories. This is especially true of some of the Romantic Naturphilosophen, such as Gottfried Reinhold Treviranus (1776-1837) and Friedrich Schelling (1775-1854), who would eventually argue for the existence of an autonomous field of biology precisely on the basis of such vital forces, which would distinguish the life sciences from the physical sciences.

The word ‘evolution’ did not disappear with the waning of preformationism, however. The vernacular meaning of the word, which could be used to describe almost any connected series of events, had by that time been used by some epigeneticists as well to describe embryonic development in general. In addition, confusion had arisen over its technical meaning in embryology. The French physiologist Etienne Reynaud Serres (1786-1868), for example, believed that Haller had taken a middle position between preformation and epigenesis, with ‘evolution’ signifying that the embryo “passed through diverse stages which were no longer its original state; in a word, it changed”.[3] Haller, however, had done no such thing.

Interestingly, some naturalists had begun to use the word in another sense as well, namely to describe progressive species development. The Swiss naturalist Charles Bonnet (1720-1793), for instance, had connected the idea of preformed embryos with that of the chain of being, stating that ever more complicated species developed over time from preformed germs. Both processes he denoted with the word ‘evolution’.[4]

Species Evolution

In the early 19^th century, with the fall of preformationism, epigeneticists also began using the word more generally, but now to signify not preformed but progressive development towards increasing complexity. This was expressed in various theories of recapitulation, in which the embryo would pass through the adult stages of lower species. One example is Friedrich Tiedemann (1781-1861), the German physiologist who held that “the entire animal kingdom has its evolutionary periods, similar to the periods which are expressed in individual organisms.”[5]

Still, ‘evolution’ was used only occasionally to refer to the transmutation of species during the first half of the 19^th century. It seems to have been the British philosopher Herbert Spencer (1820-1903) who eventually popularized the use of the word for this purpose. In an explicit embryological metaphor that closely resembles epigenetic formulations of development, he defined evolution as “a change from an indefinite, incoherent homogeneity, to a definite, coherent heterogeneity; through continuous differentiations and integrations.”[6]

Charles Darwin (1809-1882), far less keen than Spencer to talk in such bluntly progressivist terms, disliked the word ‘evolution’ and preferred his ‘descent with modification’. Perhaps because other biologists did like progress, or perhaps simply due to its being a more succinct term, ‘evolution’ stuck. Thus, the word travelled from embryology right into theories of species transformation, in large part because to various naturalists the two phenomena seemed so closely related.

Recapitulation

This relatedness can be seen most clearly in the many theories of recapitulation that popped up all through the late 18^th and the 19^th century. In part II I will treat several theories of recapitulation, varying widely in both content and radicalness, but all implying that embryonic development replays, mirrors, echoes, or at any rate strongly resembles the transformation of species.

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Robbert J Striekwold is a MSc student in the History and Philosophy of Science programme at Utrecht University, specializing in the history and philosophy of evolutionary theory. He is writing his thesis on conceptual issues in modern evolutionary developmental biology.