Darwin pointed out that the Madagascar Star orchid has a spur 30 centimeters (about a foot) long, with a puddle of nectar at the bottom. Now, evolution says that nectar isn't free. Creatures that drink it pay for it, by carrying pollen away to another orchid. For that to happen, the creature must rub against the top of the spur. So, Darwin concluded that the spur had evolved its length as an arms race. Some creature had a way to reach deeply without shoving itself hard against the pollen-producing parts. Orchids with longer spurs would be more likely to spread their pollen, so Darwin's gradualistic scenario applied. The spur would evolve to be longer and longer. From the huge size, the creature must have evolved in return, reaching deeper and deeper. So, he predicted in 1862 that Madagascar has a species of hawkmoth with a tongue just slightly shorter than 30 cm.
The creature that pollinated that orchid was not learned until 1902, forty years later. It was indeed a moth, and it had a 25 cm tongue. And in 1988 it was proven that moth-pollinated short-spurred orchids did set less seed than long ones.