552 THE WONDER OF LIFE 



elsewhere. Dr. Stuart Thomson points out that many 

 colours occur on different specimens purple, red, green, 

 grey, intermediate between purple and grey, intermediate 

 between green and purple, pink, lilac, and so on. The 

 coloration is variable. The investigator has tested various 

 utilitarian interpretations, and finds them all untenable. 

 The coloration is not protective, or aggressive, or for 

 warning, or connected with sex. Its meaning is to be 

 sought for in the internal physiological processes (probably 

 respiratory or excretory) of the animals themselves. To 

 this sound conclusion we have simply to add that it is 

 conceivable that in the future the coloration might be 

 caught up into the service of the sea-urchin's general life, 

 and utilized, let us say, for protection. 



REGENERATION OF LOST PARTS 



From ancient times something has been known of the 

 regenerative capacity of living creatures. The pruned 

 tree regrows its branches and a small cutting can re-grow 

 the entire plant. The hair 

 that is shaved off soon grows 

 again sometimes reproducing 

 millions of cells every day, and 

 the stag's antlers fall off and 

 are re -grown every year. But 

 it was not till the eighteenth 

 FIG. 89.-" Monster" form of century that scientific attention 

 Hydra 1 grisea, with eight was focussed on the remarkable 

 heads and two bases. (After , . T , ,. _,, , m 



Roesel von Rosenhof.) facts. It was then that Trem- 



bley discovered the freshwater 

 polyp and called it Hydra, after the monster with which Her- 



