A creature must balance perfecting a skill or trait (building up legs to run faster) against experimenting with new traits (wings).

生物必须在完善现有技能、特质(练腿力以便跑得更快)与尝试新特质(翅膀)之间作取舍。

youdao

After this I ran faster than my legs could take me out of his house, forgetting my friends until I stopped across the street and noticed they were right behind me.

见此情景，我撒腿就跑，都把朋友们忘在脑后了，等到我跑出房子，在对面街上停住脚步时，才发现他们都跟在我身后跑出了房子。

youdao

If I had longer legs, I would run faster.

假如我腿再长一点，我就能跑得更快了。

youdao

Bolt's reaction time was typically slow, with Gatlin, Blake and Gay faster out of the blocks, but the legs in lane seven were starting to drive.

CNN: Olympic Bolt: Witness report of historic 100m