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Sedgwick's Two Challenges

Explore Evolution asserts that in 1894, Adam Sedgwick challenged Darwin's two claims about embryos, 1) early embryos of related organisms are more similar than adults and 2) the younger the embryos, the greater the resemblance. A comprehensive comparison of vertebrate embryos does not support Sedgwick's challenges about the similarity of embryos. Explore Evolution also neglects to mention that the section of Sedgwick's paper quoted by Explore Evolution is actually challenging von Baer's Law (that development proceeds from a more general to a more specific morphology such that taxa-specific features are added later in development), and not Darwin or Haeckel.

From Explore Evolution:

In 1894, Adam Sedgwick, an embryologist at Cambridge University, challenged Darwin's two claims: 1) that vertebrate embryos were more alike than the vertebrate adults, and 2) that the younger the embryos, the greater the resemblance.
Explore Evolution, p. 68

Adam Sedgwick's critique was directed not against Charles Darwin, but Karl von Baer:

The generalization commonly referred to as v. Baer’s law is usually stated as follows: - Embryos of different members of the same group are more alike than the adults, and the resemblances are greater the younger the embryos are examined.
Adam Sedgwick, 1894. "On the Law of Development commonly known as von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development," Quart. J. Microscopy , 36, p. 35

Darwin did think that the Karl von Baer's observations provided strong support for descent with modification. However, this does not merit the inaccurate statement that von Baer's claims can be attributed to Darwin.

Explore Evolution cites Sedgwick to challenge the first of von Baer's claims:

Even the embryos of "closely allied animals," such as chickens and ducks, display specific differences very early in development. "I can distinguish a [chicken] and a duck embryo on the second day," he wrote.
Explore Evolution, p. 68

A more recent and comprehensive analysis of embryogenesis of poultry shows that morphological differences between two day duck and chick embryos are only because duck embryonic development is slower than chick embryonic development. Otherwise, their development is "nearly identical". Hence, Sedgwick's first challenge to von Baer is not supported.

Chicken, turkey, Japanese quail, and Pekin duck blastoderms from oviductal eggs showed differences in the rate of development that were inversely correlated with egg size. … Although it is recognized that the temporal rate of development will differ between different species and strains, the external features of any embryo in any given stage will be nearly identical.
Sellier, et al., 2006. "Comparative Staging of Embryo Development in Chicken, Turkey, Duck, Goose, Guinea Fowl, and Japanese Quail Assessed from Five Hours After Fertilization Through Seventy-Two Hours of Incubation." J. Appl. Poult. Res., 15, p. 219

Explore Evolution also cites Sedgwick to challenge the second claim of von Baer, that younger embryos of different groups have a greater resemblance than older embryos.

Comparing the embryos of a chicken and shark, he continued, "There is no stage of development in which the unaided eye would fail to distinguish them with ease."
Explore Evolution, p. 68

Sedgwick does observe clear differences between these dogfish and chick embryos.

According to law of v. Baer these embryos (fowl and dogfish) ought to be closely similar in the younger stage. Do these embryos, developing under similar conditions, conform to the law? Superficially, clearly not. There is no stage of development in which the unaided eye would fail to distinguish between them with ease- the green yolk of one, the yellow yolk of the other; the embryonic rim and blastopore of the fish, the absence of these in the chick; the six large gill-slits bearing gills on the one hand, the four rudimentary clefts on the other; the small head, straight body and long tail, as opposed to the enormous head, cerebral curvature , short tail, and so on.
Adam Sedgwick, 1894. "On the Law of Development commonly known as von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development." Quart. J. Microscopy, 36, p. 36

However, Sedgwick also admits that there are "striking similarities" between these embryos which are not found in adults.

These embryos are not closely similar, but it is maintained that the law is justified by certain remarkable features of embryonic similarity which the adults do not exhibit, and of which the most important are the presence in the chick of pharyngeal clefts, a tubular piscine heart and a similarity in the arrangement of the cardiac arterial system, a cartilaginous endo-skeleton, oro-nasal grooves and a notochord. Now I freely admit that these are striking similarities, but I question whether they are sufficient to justify the law of v. Baer.
Adam Sedgwick, 1894. "On the Law of Development commonly known as von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development." Quart. J. Microscopy, 36, p. 36

Explore Evolution asserts that Sedgwick had successfully disproved von Baer's second claim that younger embryos show a greater resemblance to each other than older embryos. However, when considering the major differences between the fully developed dogfish and chick embryos (presence or absence of limbs, feathers, teeth etc.) as well as the shared features of these embryos at earlier stages that are not found in adults (heart structure, cardiac arterial system, notochord etc.), it is apparent why biologists consider the early embryos to be more similar than later embryos. Interestingly, Explore Evolution does not ask students to specifically examine the early and late dogfish and chick embryos shown in Figure 4.2.