The Origin of Species
"If it could ever be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down."
"We have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organization. This resemblance is often expressed by the term 'unity of type'; or by saying that the several parts and organs in the different species of the class are homologous .. What can be more curious than the hand of a man, formed for grasping, that of the mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?"
"If we suppose that the ancient progenitor, the archtype as it may be called, of all mammals, had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain signification of the homologous construction of the limbs throughout the whole class." Ch 13
[This was discussed on p106-7 of Bethell's "Darwin's House of Cards"]
"The bones of a limb might be shortened and widened to any extent, and become gradually enveloped in thick membrane, so as to serve as a fin, or a webbed foot might have all its bones, or certain bones, lengthened to any extent, so as to serve as a wing; yet in all this great amount of modification there will be no tendency to alter the framework of bones or the relative connection of the several parts." Origin of Species,2nd ed p433-435
[Bethell really pounces on all these assertions which are really like the "is to ought" errors that Hume attacks. p107"Darwin's House of Cards"]
Letter to William Graham, 3 July 1881
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