ANOTHER TECHNIQUE UTILISED BY DARWIN: COMPARISON OF SPECIES
Another feature relied upon by Darwin to substantiate his theory, was a comparison of the different species’ basic structure[1]. One finds certain basic parts or forms across species, natural selection then accounting for the infinite diversity in structure and function within them. Thus, the skeletons of all mammals are identical, but their individual bones are different, and, give or take the odd bone here and there, the same set of 28 bones, which can be labelled with the same names, is found across all mammals. The emphasis is not on the bones themselves but on the order in which they line up. The pattern of resemblances among the skeletons of modern animals is exactly the pattern we should expect if they are all descended from a common ancestor, some of them more recently than others. The tail of a spider monkey is homologous[2] to my coccyx, just as the enormously long and strong wing bone of a pterodactyl is homologous to my little finger, and a horse’s hoof is homologous to the fingernail of your middle finger or the toenail of your middle toe, all modified to do a different job. Similarly with crustaceans - the crustacean exoskeleton is invariant across all crustaceans while the individual ‘tubes’ vary.
Considerations such as these proclaimed to Darwin “plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments”[3].
Source for illustration: Richard Dawkins, Greatest Show
[1] See for example Chapter 6, Mutual Affinities of Organic Beings. See also Dawkins, Greatest Show, Ch 6, “The tree of cousinship”, p 285 ff.
[2] Homologous resemblances are those inherited from the shared ancestor. The word ‘analogous’ is used to describe resemblances due to shared function, not ancestry – a bat wing and an insect wing are analogous, but a bat wing and human arm are homologous): Dawkins, Greatest Show, 313.
[3] Darwin, On the Origin of Species, conclusion of Chapter 13 on the ‘Mutual Affinities of Organic Beings”, p 400. See also GS, Ch 10.
Another feature relied upon by Darwin to substantiate his theory, was a comparison of the different species’ basic structure[1]. One finds certain basic parts or forms across species, natural selection then accounting for the infinite diversity in structure and function within them. Thus, the skeletons of all mammals are identical, but their individual bones are different, and, give or take the odd bone here and there, the same set of 28 bones, which can be labelled with the same names, is found across all mammals. The emphasis is not on the bones themselves but on the order in which they line up. The pattern of resemblances among the skeletons of modern animals is exactly the pattern we should expect if they are all descended from a common ancestor, some of them more recently than others. The tail of a spider monkey is homologous[2] to my coccyx, just as the enormously long and strong wing bone of a pterodactyl is homologous to my little finger, and a horse’s hoof is homologous to the fingernail of your middle finger or the toenail of your middle toe, all modified to do a different job. Similarly with crustaceans - the crustacean exoskeleton is invariant across all crustaceans while the individual ‘tubes’ vary.
Considerations such as these proclaimed to Darwin “plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments”[3].
Source for illustration: Richard Dawkins, Greatest Show
[1] See for example Chapter 6, Mutual Affinities of Organic Beings. See also Dawkins, Greatest Show, Ch 6, “The tree of cousinship”, p 285 ff.
[2] Homologous resemblances are those inherited from the shared ancestor. The word ‘analogous’ is used to describe resemblances due to shared function, not ancestry – a bat wing and an insect wing are analogous, but a bat wing and human arm are homologous): Dawkins, Greatest Show, 313.
[3] Darwin, On the Origin of Species, conclusion of Chapter 13 on the ‘Mutual Affinities of Organic Beings”, p 400. See also GS, Ch 10.