Inferring and Explaining

73 Immediately following this quote, Darwin articulates as a question the data regarding the forelimbs in mammals that is perhaps, for my students at least, the most convincing bit of evi- dence for common descent. What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of a horse, the paddle of the porpoise, and the wing of a bat, should all be constructed on the same pattern, and should include the same bones in the same relative positions? 10 Pretty darn curious, wouldn’t you agree? e 5 . Morphological commonalities Embryological Facts As Darwin remarks several times in his discus- sion of embryos, just as the remarkable similar- ity in the bones in the forelimbs of mammals require an explanation, curious features of embryos also must be accounted for. How, then, can we explain these several facts in embryology,—namely the very general, but not uni- versal diference in structure between the embryo and the adult;—of parts of the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike;—of embryos of diferent 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 appar- ently having sometimes a higher organization than themature animal, intowhich it is developed. I believe all these facts, as follows, on the view of descent with modifcation. 11 Give me a common ancestor, Darwin seems to say, and I can explain the circuitous route, with many detours, from egg to adult in animal development—whyyoungbaleenwhales develop teeth, why land-living vertebrates (including ourselves) go through a gill-arch stage, and why higher vertebrates have a notochord. Tere is no obvious reason why, for instance, the wing of a bat, or the fn of a porpoise, should not have been sketched out with all the parts in proper propor- tions, as soon as any structure became visible in the embryo. 12 darwIn and Common desCent e 6 . Embryological oddities I must tell you here that were you to Google “Darwin, embryology,” youwould fnd that some of the sites are highly critical of Darwin and argue that his appeals to embryology have been discredited in modern biology. Tis isn’t really true, but we can make sense of these, I believe sincere, criticisms. Part of the problem Darwin faced in his section on embryologywas that cen- turies of thought had tied embryonic develop- ment to the static scala naturae that we discussed earlier. These scientists believed that each stage in embryonic development represented an earlier, nonchanging stage in the hierarchy of life. Darwin, of course, completely rejected this view, but it remained part of the common (mis)understanding within embryology. Even more problematic, though, was that many scientists sympathetic to biological change