Types of comparison
Biologists distinguish analogous traits from homologous traits. Analogous traits are similar, because of convergent adaptation towards similar environmental conditions. An analysis of the fine structures of these traits, however, will show differences proving that they have been derived from different ancestral traits. The similarity is only superficial. The eyes of vertebrates and octopuses often serve as an example of analogous traits. Although they look extremely similar, superficially, analysing the fine structure reveals, for example, that the innervation of the retina is inverse in vertebrates, but not in octopuses.
Homologous traits can look similar or different, depending on the similarity or difference in environmental conditions to which they have been adapted. An analysis of their fine structure, however, will show identities proving that they have been derived from the same ancestral trait. The standard example for homologous traits are the limbs of vertebrates. Although they can look as different (divergent) as the wings of bats and birds or the legs of horses and humans, the fine structure (of bones, tendons, muscles) reveals that they have been derived from the same ancestral limb.
With this distinction, we can categorise comparisons as follows. If a comparison highlights differences in effects but does not compare the underlying causes, it will be a superficial contrast. If it highlights similarities in effects but does not compare the underlying causes, it will be a superficial analogy. If it highlights similarities in effects but also shows that these are due to the different underlying causes, it will be a deep analogy. Finally, if effects are similar or divergent, but the underlying causal machineries are identical, it will be a homology.
Categories of comparisons
| superficial analogy | deep analogy | superficial contrast | homology | |
effects | similar | similar | different | similar or divergent |
causes | -- | different | -- | identic |
M. L. Vilmorin's contrast
The house of Levêque de Vilmorin is a famous family of botanists, seed dealers and breeders (note: one French synonym for breeder is "sélectionneur"). Its history reaches back to the Parisian seed store of Claude Geoffroy and her husband Pierre Andrieux, who were the chief seed suppliers for king Louis XV. Their daughter, Jeanne Marie Adélaïde Andrieux (1756-1836), married Philippe Victoire Levêque de Vilmorin (1746-1804). They created the Vilmorin-Andrieux house (1775), which grew into the Vilmorin-Andrieux company (1815). Today Vilmorin & Cie is, according to its own website, the fourth biggest seed producer in the world.
At 6 January 1847, M. Trochu wrote a letter to L. Vilmorin, describing his experiments in trying to fix a variety of Gorse without thorns, which he had found near his house. He Failed but inspired M.L. Vilmorin to write an article in turn (Vilmorin, M.L. 1851. "Notes sur un projet d'exprérience ayant pour but de créer une variété d'ajonc sans épines se reproduisant par graines; par M. L. Vilmorin, membre correspondant de la Société Industrielle, à Paris. Bulletin de la Société Industrielle d'Angers 22: pp. 253-261).
After describing the efforts of his colleague, Vilmorin speculates about species and varieties, about the forces that keep the species fixed in nature, and of the spectre of breeding and fixing new varieties. Vilmorin thought that a balance of forces prevailed in nature keeping the species close to its type, but that humans could upset this balance in favour of the force that pulled varieties away from the type. He therefore thought of these two forces as a centripetal and a centrifugal one pulling away or towards the type respectively. He also referred to these "forces" as "laws," calling the centripetal one the law of atavism (or similarity to species type) and the centrifugal one the law of individual variation or idiosyncrasy. Here's my translation:
"From what we know about the power of nature in the law of individual variations, it must look very likely that there exists on the extent of the Bretagne any number of Gorse individuals without thorns in the middle of an immense number of thorny individuals. However, it is sufficient that one of these individuals to print direct descent from seed a little more pronounced character, and as any part of its products were unarmed like him, that it was then possible to arrive quickly enough by a well-understood selection, completely free the new breed. But if we think that individuals more or less devoid of thorns which were met so far were from thorny parents, we will conceive the chance to get spineless relatives of products that are likewise is necessarily slightly larger, and that this opportunity will increase as the number of successive generations will grow for the modified plant.We can here see a specimen of a what might be called a physicalistic 'force paradigm.' Herbert Spencer' Synthetic Philosophy also had many pairs of forces keeping each other in moving equilibria (see here). In Vilmorin's case, these forces are called Atavism and Idiosyncrasy (the law of individual variations). After these pseudo-Newtonian speculations about pseudo-forces, however, he arrived at the insight that artificial selection can transform species (see quote above).
If we consider a seed when planted and will create a new individual, we can look at it as attracting two distinct and opposing forces, regarding the characters that will present the plant that must be born. These two forces, which act in the opposite direction and balance each other results in the fixity of species that can be considered as follows: The first, or centripetal force, is the result of the law of similarity for children to fathers, or atavism; its action results in maintaining the species within the assigned limits of variation, while the differences is produced by the opposing force.
The former, or centrifugal force, resulting from the law of idiosyncrasy that each individual in a species, although it may be supposed born of an individual (or couple), features unique differences that constitute its own physiognomy and produce the infinite variety in unity that characterizes the works of the Creator.
For simplicity, we first consider atavism as constituting a single force; but if you think about it, we will see that it is rather a bundle of forces acting approximately in the same direction and is composed of the individual attraction of all ancestors. In order to facilitate the understanding of the action of this force, however, we first have to abstractly consider the force of similarity in/as the mass of ancestors, which can be considered to constitute the attraction of the species type, and to which we will reserve the name of atavism; then separately and in a more special way, the attraction or force of similarity to the direct father, which, less powerful but closer, tends to perpetuate in the child the proper characters of the immediate parent.
As long as the father is not appreciable far from the type of the species, these two forces act in parallel and blend, and changes that may occur in this case, by virtue of the law of idiosyncrasy, may occur in all directions without altering any particular. It is no longer so if the direct parent is significantly distant from the species type; the force of similarity to the direct parent now combines with the one of individual variation leading to excessive deviation in the direction of the resultant of the two forces, or, if one prefers, the new changes then radiate, not around the species type as center, but around a point placed on the line, which separates the type from the first deviation [variant] obtained.
Abandoned to nature, individual variants almost always die in the overflowing mass of individuals that it [nature] sacrifices continually. Hence the fixity of natural species. But obtained by humans, these variations are protected; their descendants multiply; while obeying the more complex laws governing now, they produce the many changes he was able to fix for his use. It was then also the influence of man, choosing to multiply through offspring only the modified individuals, counterbalancing, by constant efforts, the constant force of atavism, and comes to free or fix the modified races." (p. 255-257)
That is, humans can protect variants or sports from nature that would otherwise eliminate them. As Vilmorin also thinks that more complex laws are governing under the human regime than under nature, he sees a contrast between artificial and natural selection, not an analogy or homology.
Naudin's homology
A reprint or extract of Vilmorin's article from 1851 was published one year later as: Vilmorin, L. 1852. "Ajonc sans épines1. Notes sur un projet d'expérience ayant pur but de créer une race d'Ajonc sans épines se reproduisant de graines." Revue Horticole, Ser. 4, Tome 1: 22-29. The footnote (1) in the title states that this article is an extract from the earlier one. This extract, inspired Charles Naudin (1852 Considérations philosophiques sur l'espéce et al variété. Revue Horticole, Ser. 4, Tome 1: 102-109) to ponder the species problem in turn. He opened as follows:
"No doubt, the readers of the Revue Horticole have read with interest, in the issue of January 16, an article of M. L. Vilmorin about a variélé of thornless Gorse, whose fixation would be of great importance for agriculture, where this clever experimenter develops a theory already confirmed by the experience of the possibility of creating, in species such as nature provides us with, varieties, races, or even new artificial species most directly relevant to our needs. This theory, we say, is confirmed by experience; strictly speaking, it is a statement of the methods used empirically for centuries, and those used today by horticulturists almost instinctively, and without really realizing it, to obtain new varieties in useful and ornamental species that are introduced every day in our gardens." (p. 102f)Unlike Vilmorin, Naudin saw natural and artificial selection as homologous in their causal machinery a with similar effects:
"We do not think that Nature has made its species in a different fashion from that in which we proceed ourselves in order to make our varieties; or better, we carried it's [Nature's] process into our practice." (p. 104)Naudin even mentions Lamarck, praises and criticises him, and improves his scala naturae into a tree of life (p. 105f). Thereafter follows a long discussion of the implications for classification.
"Such is, in our ideas, the course followed by nature; like us, it wanted to form races appropriate for their needs; and with a relatively small number of primordial kinds, she gave birth in succession and at various times, to all plant and animal species that inhabit the globe." (p. 104)
"Nature has operated on an immense scale and with immense resources; we, on the contrary, we do so with extremely limited means; but between its processes and ours, between his results and those we get, the difference is in any amount; between its species and those we create, there are only the more and less."
Conclusion
Now, Wallace and Darwin had not yet published and it is highly unlikely that the French horticulturist Naudin had read the book On Naval Timber and Arboriculture by Matthew (1831). Furthermore, Naudin's comparison observes that the effects of natural and artificial selection are similar and claims that this is due to a homology of the causal relations underlying both processes. Matthew (1831), on the other hand, observed that effects of natural and artificial selection are in conflict with each other and did not explicitly compare the causal machineries of both processes. His proposal is either a superficial contrast or an implied homology of causes with divergent effects.
Therefore, it is more likely that Naudin's inspiration for the strong homology with similar effects came from Lamarck's observation that nature and culture had similar effects that were due to homologous causes (see next post).
