From the 1968 lecture programme:
A flea, a hare, a man, and a horse all can jump about the same height, if we measure fairly. When we change length scale we tend to forget the quantity of time; that in all motions the scale of the moving object plays a part which is often surprising. The motions of micro-organisms and of the mist droplets in cloud reflect, above all, their small size. We extend this idea to study walking, running, floating, swimming, by large animals and small, and in man-made vehicles of all kinds.