Talk:Von Baer's laws (embryology)

Darwin's attitude to the laws

@Chhandama: In this edit you added the claim that Darwin was "the most important defender" of von Baer's theory. You quoted The Origin in order to support this. However, the quote seems rather to support the recapitulation theory (in e. g. the formulation of Haeckle) than von Baer's. Moreover, the context seems to support this view. The full page 338 reads

Agassiz insists that ancient animals resemble to a certain extent the embryos of recent animals of the same classes; or that the geological succession of extinct forms is in some degree parallel to the embryological development of recent forms. I must follow Pictet and Huxley in thinking that the truth of this doctrine is very far from proved. Yet I fully expect to see it hereafter confirmed, at least in regard to subordinate groups, which have branched off from each other within comparatively recent times. For this doctrine of Agassiz accords well with the theory of natural selection. In a future chapter I shall attempt to show that the adult differs from its embryo, owing to variations supervening at a not early age, and being inherited at a corresponding age. This process, whilst it leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more difference to the adult.

Thus the embryo comes to be left as a sort of picture, preserved by nature, of the ancient and less modified condition of each animal. This view may be true, and yet it may never be capable of full proof. Seeing, for instance, that the oldest known mammals, reptiles, and fish strictly belong to their own proper classes, though some of these old forms are in a slight degree less distinct from each other than are the typical members of the same groups at the present day, it would be vain to look for animals having the common embryological character of the Vertebrata, until beds far beneath the lowest Silurian strata are discovered—a discovery of which the chance is very small.

Darwin argues rather explicitly for why the embryonic features actually should have developed as (adult) features of extinct forms; namely, because they elsewise could not have been favoured by natural selection. (This was a mistake of the logic, as later evolutionists have shown.) Neither here, nor anywhere in that edition of The Origin, von Baer is mentioned. (Darwin added his systematic exposure of his precursors in a later edition.)

Thus, I think that this quote should be presented as severe criticism of von Baer's theory, rather than a support, if you do not see any reason for not doing it. Best, JoergenB (talk) 16:15, 3 October 2020 (UTC)

Yes, a bit weird: Darwin was here arguing in Haeckel's direction, not von Baer's. This is remarkable, because elsewhere Darwin critiques recapitulation, and correctly says that adult animals don't resemble other animals' embryonic stages. Perhaps it would be best just to remove the passage. Failing that, it certainly needs to be balanced with more representative material. Chiswick Chap (talk) 16:34, 3 October 2020 (UTC)

I'll reread the seemingly relevant part of my 'Origin' (based on a later edition), and see what impression I get. JoergenB (talk) 22:10, 3 October 2020 (UTC)

Strange and contradictory as it may seem, Darwin did support the law, and this is the common scholarly interpretation. I have added information in the text. Chhandama (talk) 06:42, 4 October 2020 (UTC)

I reread the historic sketch and chapters 9, 13, and 14 in my copy of 'The Origin'. I must modify my claim that that is a later edition; it does contain "an historical sketch" and an index which are included from later editions, but apart from that it has the text from the first one. Thus, I've read what Darwin wrote about von Baer in the "sketch", but not yet read his more thorough discussion of the laws.

@Chiswick Chap: As far as I can see, Darwin only claims that no recent adult animal resembles any other recent animal's embryonic stage. Thus, no contradiction has to exist, if von Baer did not encompass the general notion of evolution. Darwin notes that quite a lot of biologists were prepared to accept the existence of a rather small scale and partial evolution, where either some apparently different species within the same genus were not "true species", since they had evolved from a common ancestor by means of successively more different varieties, or in fact in general only a separate creation for each genus, from then all the species within that genus had evolved. For us it may seem a bit strange that some biologists simultaneously could accept the limited but not the general evolution. However, we may remember that even today various kinds of Creationists often accept what they call microevolution but not the generalisation to Darwin's concept.

Also, von Baer seems to have referred to 'higher' and 'lower' organisms with in a taxon; but with the attitudes in these days, I do not think this should be interpreted as 'preceding' and 'succeeding' forms, but more to coexisting forms he judged to be more 'primitive' or 'advanced', respectively.

Thus, Darwin may very well have argued that on the one hand no living forms represent the embryonic form of another living organism, but on the other hand the adults some extinct organism (often rather hard to find due to the incompleteness of the geological record) indeed should have been rather close to such embryos. There is no logical contradiction between a "present taxon subordination von Baer" and a "prehistoric recapitulation Meckel-Serres" theory.

From what I now read in 'The Origin', I think that this is not very far from Darwin's opinion, but that he indeed also recognised the possibility that also the embryonic phases could have been modified due to natural selection, to some extent. This yields the obscurity he refers to in his summaic statement in chapter 14:

Embryology will reveal to us the structure, in some degree obscured, of the prototypes of each great class. (Darwin, Charles (1859). On the Origin of Species. London: John Murray. p. 486.)

Darwin recognises that modification of non-adult forms may occur, when these juveniles must fend for themselves, as most larvae of insects with full metamorphosis. However, from a modern perspective, he seems to underestimate the impact of such modifications, for at least two reasons.

• Darwin argues that e. g. the stripes of lion cubs and the spots on the plumages of juvenile thrashes (also in species where the adult is not spotted) must be inherited from the colourings of some adult ancesters, since in neither case natural selection would affect these youngsters, since their parents provide their food, whence they were exempt from the struggle for survival. However, struggle for survival encompasses more than just food searching; and the survival of both the cubs and the chickens probably is enhanced by protective colourings. (I've heard that the greatest danger for lion cubs often is adult lions not belonging to their own pride; but this should not lessen their need for camouflage.)
• I think that Darwin also underestimates the selective pressure with respect to early embryonic changes yielding phenotypical changes in the adults. JoergenB (talk) 13:42, 9 October 2020 (UTC)

• What a curious and complex position. We need a citation if this is to move into the article, however. Chiswick Chap (talk) 13:45, 9 October 2020 (UTC)

Yes. However, I think that we first of all should try to find out what von Baer's position was. JoergenB (talk) 13:53, 9 October 2020 (UTC)