Whereas purposeful men could make their way across a narrowbridge, the slow diffusion of plant and animals would require an avenue as a continent and available for ages at a stretch.
Kennedy missed the main road and, driving at speed down a narrow dirt track, ran off a narrowbridge.
肯尼迪错过了主干道，飞快地冲下一条狭窄的泥土路，冲出一条窄桥。
3
The physicists give a simplified example of how this can happen: trying toreach a destination either by using a short but narrowbridge or a longer butwide motorway.