They were trained to run.

They, and their descendants after them, pair following on pair; first with slow-turning wheels as in squirrel cages, the wheel inexorably going, machine-driven, and the luckless little gluttons having to move on, for gradually increasing periods of time, at gradually increasing speeds.

Pair A and their progeny were sheltered and fed, but the rod was spared; Pair B were as the guests at "Muldoon's"-- they had to exercise.

With scientific patience and ingenuity, he devised mechanical surroundings which made them jump increasing spaces, which made them run always a little faster and a little farther; and he kept a record as carefully as if these little sheds were racing stables for a king.
Several centuries of guinea-pig time went by; generation after generation of healthy guinea-pigs passed under his modifying hands; and after some five years he had in one small yard a fine group of the descendants of his gall-fed pair, and in another the offspring of the trained ones; nimble, swift, as different from the first as the razor-backed pig of the forest from the fatted porkers in the sty.
He set them to race--the young untrained specimens of these distant cousins--and the hare ran away from the tortoise completely.
Great zoologists and biologists came to see him, studied, fingered, poked, and examined the records; argued and disbelieved--and saw them run.
"It is natural selection," they said.

"It profited them to run." "Not at all," said he.