Rather than presenting an account of how embryology is studied in the 21st century, the "Embryology" chapter concludes by exhorting students to pretend they are jurors in a court case against evolution. It's unclear what the charges might be, but it is certain that the students would be diverted from a fair verdict by the chapter's studious avoidance of current science in the field.
Rather than a scientific argument, the book offers a philosophical charge, that evolution has been rendered unfalsifiable. This charge rests on a misrepresentation of how science works, and what evolutionary developmental biologists actually claim. It is one thing to argue that there is no evidence which currently falsifies common ancestry, and quite another to say that no such evidence could ever exist. Scientists agree to the former statement, but not the latter.
Explore Evolution argues that if common descent predicts both similarity and dissimilarity of embryos, it is impossible to challenge the theory. The similarity of embryos is best explained by common descent. The dissimilarity of embryos can be explained by environmental adaptations under natural selection, and while it can cause scientists to re-evaluate particular claims of common ancestry, no embryological evidence now available would cause scientists to reject universal common ancestry.
Explore Evolution insists
By arguing that common descent predicts both embryonic similarity and dissimilarity, Darwinists have effectively made it impossible to challenge the theory with counterevidence. When the case is stated this way, common descent would be consistent with whatever we observe in embryos.Explore Evolution, p. 70
In this claim, Explore Evolution fails to distinguish two main threads of modern evolutionary biology; common descent and the role of natural selection in adaptation. As noted earlier, Darwin realized that embryos would show differences if they had adaptations to different environmental circumstances. Perhaps the most important adaptation is the amount of yolk, which affects the egg size and the amount of stored nutrition available for the developing embryo. One of the most striking differences is between animals that undergo metamorphosis from a feeding larval stage and animals that undergo direct development, bypassing the larval stage. The comparative analyses of sea urchin embryos and larvae has shed light upon the adaptive strategies of these different modes of development.
Observations of a [sic] sea urchin larvae show that most species adopt one of two life history strategies. One strategy is to make numerous small eggs, which develop into a larva with a required feeding period in the water column before metamorphosis. In contrast, the second strategy is to make fewer large eggs with a larva that does not feed, which reduces the time to metamorphosis and thus the time spent in the water column. The larvae associated with each strategy have distinct morphologies and developmental processes that reflect their feeding requirements, so that those that feed exhibit indirect development with a complex larva, and those that do not feed form a morphologically simplified larva and exhibit direct development.Smith, Zigler and Raff, 2007. "Evolution of direct-developing larvae: selection vs loss." Bioessays, 29:6, p. 566
Significantly, these different modes of development are found in closely related species who diverged relatively recently, about 4 million years ago, and have occurred in multiple instances in other echinoderm lineages. The comparison of indirect and direct development has demonstrated that the earliest stages of development are relatively plastic (Raff ref).
Shared features of organisms are normally most parsimoniously explained by inferring common descent. The support for common descent, which is disputed by Explore Evolution but accepted by the vast majority of biologists, arises from the independent convergence of evidence from a wide variety of fields including biogeography, biochemistry, molecular biology, and embryology. One such shared feature of primate embryos, including human, is a tail. Primates which lack a tail as adults, such as humans and chimps, resorb the tail during later embryogenesis. Primates which have a tail as adults, do not resorb the embryonic tail. Phylogenetic analysis have unambiguously demonstrated that the ancestors to primates were tailed. The vast majority of biologists would consider human embryonic tails to be best explained by common descent. How would Explore Evolution explain it to students?
Explore Evolution claims that many scientists who criticized Haeckel's embryos still support common ancestry; but students, as good jurors, should keep an open mind. Students are learners, not jurors. Their science class is a chance to gain enough context to continue their science education in college and graduate programs, where they will get the background necessary to challenge well-established science. To suggest that students should reject such science at the beginning of their scientific careers is irresponsible.
Keeping an open mind is a virtue when coupled with skepticism and critical analysis. Such skepticism and analysis requires accurate information, which Explore Evolution fails to offer.
Asking students, at the very start of their biological studies, to keep an open mind about the views of an extremely small minority of biologists has questionable pedagogical value. This is particularly problematic because Explore Evolution suffers from so many distortions.
In Explore Evolution, students are exposed to a set of arguments from authority presented in a courtroom-like "he said/she said" fashion. This is not "inquiry-based" education, but rather a rhetorical exercise.
In science, there are not always two diametrically-opposed sides to every issue. When measuring the velocity of objects falling in gravity fields, there is no "dissenting view" or other side that can claim equal time. In areas of active scientific research, there are often more than two sides or interpretations. The courtroom-like approach of this book is an inaccurate representation of how science works.