Sentence:  In both species it is impossible for the eye to follow the <WSD>incessive</WSD> movements of the feet, and to compare them with those of other quadrupeds, but from their chrono-photographic images it is easy to see that, taking the order of the movements of the limbs as a standard, the lizards are trotting animals. 
Meaning of the word marked with <WSD> tags:  Intense and active.