THE WONDER OF LIFE 559 



shown that the regeneration is absent or very incomplete. 

 Has the absence of it something to do with the fact that 

 in those aggressive hzards that use their tail as a weapon, 

 the loss of the tail is not hkely to occur ? Perhaps a clearer 

 case is that of the Chameleon, which coils its prehensile 

 monkeyish tail round the branch. There seems to be httle 

 or no regeneration, if the tail be cut off. Has the absence 

 of it something to do with the fact that in the case of the 

 quaint Chameleon, the loss of the tail is not hkely to occur ? 

 Some of the Umitations are less readily interpreted. Thus 

 the weakly developed limbs of Siren and Proteus are not 

 regenerated, but the well- developed hmbs of the newt are. 

 A salamander will re-grow an amputated hmb if the bone 

 be cut across and not disarticulated, but in a frog the wound 

 heals without regenerating. The puzzle is why there should 

 be such differences in the regenerative capacity of nearly 

 related types. Another instructive case is that of the sea- 

 urchin which cannot regenerate anything but its spines. 

 Why is there this hmitation in a member of the Echinoderm 

 class in which the regenerative capacity is widespread and 

 highly developed ? Is it that the globular sea-urchin is 

 not subject to the same risks as a starfish or a brittle-star ? 

 Another general fact, which points the way to a theo- 

 retical interpretation, is that regeneration of internal parts 

 is very rare among backboned animals, and rare even among 

 backboneless animals except in cases hke those of earth- 

 worms and starfishes, where the loss of part of the body 

 necessarily injures the internal organs, such as the alimen- 

 tary and nervous systems. If a rabbit's spleen be removed, 

 it is not replaced ; if a fragment be left, it does not grow 

 larger. The removal of a kidney, a thyroid, a hver-lobe, 

 and so on, is not known to be followed by regenerative 



