222 DYNAMICS OF LIVING MATTER 



covered with epidermis and thus heals, but no regeneration of the lost 

 segment occurs. I have kept headless pieces of a leech alive for almost 

 a year, without any trace of a regeneration occurring. We thus see 

 that an Annehd may possess a much lower power of regeneration than a 

 vertebrate. 



It has been maintained that the power of regeneration is due to 

 natural selection, and, therefore, runs parallel to the Uability of an ani- 

 mal to injury. I do not believe that such ideas are of more value in 

 biology than they are in physics or chemistry. It is not very obvious 

 either why "nature" should care so much more to preserve the tail of 

 a salamander than the foot of the leech. Morgan has treated this sub- 

 ject exhaustively, and I refer the reader to his writings.* 



* T. H. Morgan, Regeneration, New York, igoi ; and Evolution and Adaptation, New 

 York, 1903. 



