Certainly paleontologists ...

Certainly paleontologists have found samples of an extremely small fraction, only, of the earth's extinct species, and even for groups that are most readily preserved and found as fossils they can never expect to find more than a fraction.
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 George Gaylord Simpson

Quotes from the same author

...the argument from absence of transitional types boils down to the striking fact that such types are always lacking unless they have been found.
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 George Gaylord Simpson
Species evolve exactly as if they were adapting as best they could to a changing world, and not at all as if they were moving toward a set goal.
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 George Gaylord Simpson