■WIIEEL-BEARERS. 259 



tlie same lime, all with pi-ecisely the same force, and all 

 to precisely the same extent. It follows, that before the 

 first has comjjleted its beat and resumed the erect posi- 

 tion, three or fonr others are in various degrees of flex- 

 ion, regularly graduated ; and that if the eye could 

 look laterally at such a row of cilia suddenly arrested 

 and fixed as they were, we should see their tips tracing 

 a wavy line instead of a straight one. Moreover, since 

 the bending of any cilium brings its tip nearer to its 

 successor than it was before, and this ap2)roximation in- 

 creases the farther the flexure proceeds, it follows that 

 at the bottom of each wave the tips of the cilia over- 

 lap their successors, while the spaces perpendicularly 

 above their bases are left more open by the removal of 

 their points. 



Hence, in microscopical observation of ciliated ani- 

 mals, though the individual cilia are too minute to be 

 discerned while still, we can readily discern the in- 

 creased density (and therefore opacity) of the bottom 

 of a wave, contrasted with the increased openness (and 

 therefore clearness) of the summit. So that the optical 

 eflTect is that of an alternate succession of dark and 

 light spots blending into each other. 



But as no cilium in the series is for two successive 

 moments in the same degree of flexure, and as both it and 

 all its predecessors and successors are ever urging on 

 theu* perfectly timed aud regulated course, the waves 

 are never fixed, but always gliding on with a swift but 

 beautifully even rapidity. And as it is with the waves,. 

 60 it is with their optical effect upon the eye ; the black 

 and white spots, or rather the black spots with blank 

 intervals, appear to be constantly chasing each other in 

 ceaseless race. 



