One thing that has struck me about Darwin’s Origin of Species is the anachronistic view of variation as the generator of evolutionary change. Whereas today we consider variation to be essentially a random byproduct of genetic mutation, Darwin considered variation to be a force in the direction of phylogenetic change. By contrast, modern evolutionary biology places more of an emphasis on selection, whereas variation is considered essentially random.
Take his example of Orangutans:
A part developed in any species in an extraordinary degree or manner, in comparison with the same part in allied species, tends to be highly variable.—[…] I infer also from an observation made by Professor Owen, with respect to the length of the arms of the ourang-outang, that he has come to a nearly similar conclusion. […] I can only state my conviction that it is a rule of high generality.
In other words, the greatest anatomical difference between ourang-outangs and allied species of ape (humans, gorillas, bonobos, etc…) are its long arms. And this is also precisely the part of the ape’s anatomy which varies most in individual ourang outans!
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