56o 



THE WONDER OF LIFE 



growth. Only in the case of reproductive organs does a 

 remnant grow to replace what has been removed. 



Lessona's Law. — These two sets of facts — the rarity of 

 regeneration in the internal organs of higher animals, and 



the strangely unequal distribu- 

 tion of the capacity — point to 

 a conclusion which seems to 

 have occurred to several natu- 

 rahsts from Reaumur to 

 Weismaim, and was clearly 

 summed up in 1868 by the 

 Itahan naturahst Lessona. 

 According to ' Lessona's law ', 

 regeneration tends to be well 

 marked in those animals, and 

 in those parts of animals which 

 are in the course of their 

 natural life particularly hable 

 to injury. To which, two 

 saving clauses must be added, 

 that the lost part be of some 

 vital importance, and that the 

 wound or injury be not in 

 itself hkely to be fatal. This 

 theory, which is really Dar- 

 winian, has been re-stated 

 by Weismann in the words : 

 ' the power of regeneration 

 possessed by an animal or by a part of an animal is regu- 

 lated by adaptation to the frequency of loss, and to the 

 extent of the damage caused by the loss '. 



It is evident, at once, that the lank and slender bodies 



Fig. 91. — Comet form of starfish 

 (Linckia). [After Riohters. ' 

 An arm and a portion of 

 disc regenerating the other 

 four arms. M, the mouth ; 

 RA, a regenerating arm ; OA, 

 the original arm ; AG, the 

 ambulacral groove on the 

 ventral surface of each arm. 



