568 THE WONDER OF LIFE 



normally five tarsal joints at the end of the leg, the regener- 

 ated limb has only four tarsal joints, which is believed to 

 be the ancestral number. Weismann cites the observation 

 of Fritz Miiller, that in a Brazilian shrimp, Atyoida poti- 

 morim, the long clawed forceps are replaced in regeneration 

 by the older short-fingered type of forceps, seen in the 

 allied genus Caridina. In both these cases it might be 

 said that the regeneration was economical and that not 

 more than a workable substitute for the lost part was 

 re-grown. But this cannot be the explanation, for we know 

 that regeneration will take place perfectly in half-starved 

 animals. Furthermore, there are cases where the regener- 

 ated part, though more ancestral, is not more economical 

 of material. Thus Barfurth calls attention to the very 

 suggestive fact that the four-fingered hand of the Axolotl 

 is replaced after amputation by a more typical five-fingered 

 hand. 



Weismann has suggested a speculative theory of these 

 cases. He supposes that there are regeneration-germs 

 which come to reside in areas particularly liable to injury 

 (like the cambium- cells in various parts of plants), and he 

 further supposes that these have, in their developmental 

 power, lagged a little behind the level of the part to which 

 they correspond. They are able to replace it, but not quite 

 up to the contemporary grade of evolution. It must be 

 remembered that the regeneration tends to be rapid com- 

 pared with the original development, and that the con- 

 ditions are different. Perhaps some stimulus is awanting 

 to incite the regeneration to go a step further. 



It must be admitted that in many cases the substitute 

 that replaces the lost part is not quite correct, not quite 

 up to the mark. In place of a lost leg an insect may grow 



