Haeckel on gastrulation

Just for the heck of it I went looking for what Darwin said about the similarity of embryos. The Darwin Online archive yields just 26 hits on [+similarity +embryos], all but two of them in various editions of Origin of Species. Looking through them, one finds remarks like this:

How, then, can we explain these several facts in embryology,—namely the very general, but not universal difference in structure between the embryo and the adult;—of parts in the same indivividual [sic] embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike;—of embryos of different species within the same class, generally, but not universally, resembling each other;—of the structure of the embryo not being closely related to its conditions of existence, except when the embryo becomes at any period of life active and has to provide for itself;—of the embryo apparently having sometimes a higher organisation than the mature animal, into which it is developed. I believe that all these facts can be explained, as follows, on the view of descent with modification. (OoS, 1^st Ed., pp 442-443; bolding added)

Clearly Darwin was referring to a restricted set (“within the same class”), and to a general but not universal phenomenon. He was certainly aware of differences as well as similarities, and limited it to the level of classes, not the subphyllum Vertebrata.

The two outliers in the first search were in The variation of animals and plants under domestication, where Darwin wrote

As a consequence of this the embryo, even when the parent-form undergoes a great amount of modification, is left only slightly modified; and the embryos of widely-different animals which are descended from a common progenitor remain in many important respects like each other and their common progenitor.

Again, Darwin’s statements are considerably more circumspect and qualified than Wells’s “If it were true (as Darwin and Haeckel claimed) that vertebrates are most similar in their earliest stages, then the various classes would be most similar during cleavage and gastrulation” would imply.

Searching on [+stage +similar +embryos] I get 13 hits. On a fast scan, none make the claim that Wells attributes to Darwin. I do find a similar sort of claim, though, together with a blatant “ontogeny recapitulates phylogeny” claim, in a 1903 book called An easy outline of evolution by a Dennis Hird:

Man is developed from a small cell (called the ovum or ovule), about the 120^th of an inch in diameter, which differs in no apparent respect from the ovules from which other animals grow. The embryo itself at a very early period cannot be distinguished from that of other members of the vertebrate kingdom (See Fig.7). Very wonderful is the evidence in favour of Evolution furnished by the development of man from the very beginning of his life before birth. Speaking broadly, man in his development goes through a series of changes that are the same, at different stages, as the fixed forms of the lower animals when they are full-grown. In his development from cell or egg he presents structures that are precisely like those seen in the bodies of the lower and lowest animals in their adult state. In fact, they correspond with the stages of man’s evolution in the almost infinite past. The detailed life-history of any one man shows the history of his race.

I found nothing like that searching Darwin’s writings.

Now, Darwin clearly knew that differentiation occurred during development, and that later stages showed more clearly the adult specializations that would result from the completed development process. But as far as I can tell he made no blanket statements about similarities as a function of stage starting at cleavage.

So Nick’s post shows that Haeckel was surely aware of differences, and Darwin was circumspect and qualified in his claims about similarities. Wells has over-simplified both to the point of deceptive caricature.

The final question, of course, is “Who cares?” Darwin and Haeckel wrote in the 19^th century. We’ve learned a dab about developmental processes since then.

RBH