Another difficulty Darwin faced in advocating a theory of species origin by descent from a common ancestor was the Victorian era mind-set that species were specially created and immutable or unchanging and without transitions between forms.
To support his ideas Darwin pointed to living animals that show a range of modifications of a shared structure. Using birds as an example, Darwin described the diverse development of the wing, from fully functional for flight in air, to their use as fins in an aquatic habitat, as with penguins, to functionally of no use, like the wings of the flightless kiwi.
These examples served to show the range of transitions present within a living group of animals and laid the groundwork for thinking about the cumulative effect of variations as natural selection acted over the immensity of geologic time.
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