THE WONDER OF LIFE 561 



of worms, the long arms of starfishes and brittle-stars, the 

 sprawling limbs of newts, the long tails of lizards, and 

 so on, are naturally liable to injury, and that a re- 

 generative capacity is one which natural selection would 

 foster. 



It is also evident that internal organs are much less likely 

 to be cut out than external organs are to be cut off. It is 

 also certain that visceral wounds are much more likely to 

 be fatal in Vertebrates than in Invertebrates, so that a 

 regenerative capacity in the former would be, so to speak, 

 a quality wasted, whereas in the less sensitive Invertebrates, 

 where it often occurs, it is very much in demand. If the 

 retention and specialization of the regenerative capacity 

 has been evolved as an adaptive character, it must obvi- 

 ously be restricted to cases where the injury is of a kind not 

 likely to be fatal. 



It will be observed that Lessona's law does not touch the 

 question of the origin of the regenerative capacity, nor the 

 question of how the capacity resides in the lizard's tail or 

 in the snail's horn, nor the process of re-growing a complex 

 structure from a stump. It is a theory of the distribution 

 of the regenerative capacity why is it here and not there, 

 why is it strong in some animals and weak in others, why 

 it is well marked in regard to some parts and not at all 

 marked in regard to others. The question is whether it is 

 a sufficient formula to cover the known facts in regard to 

 the distribution of the regenerative capacity. 



Testing the Theory. One way of testing the theory 

 is to inquire whether there are any or many well-authenti- 

 cated cases of the regeneration of parts which would not 

 be likely to be injured or lost in the natural conditions of 

 the animal's life. A number of difficult cases have been 



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