200 BIOLOGICAL LECTURES. 



unfortunate individuals subjected to the experiment. I need 

 not here recall numerous facts in regard to regeneration of 

 internal organs that can seldom or never (except in artificial 

 operations) be exposed to injury. 



Last year I undertook a series of experiments on the regen- 

 eration of the appendages of the hermit crab, in order to deter- 

 mine whether regeneration takes place more readily in the 

 appendages exposed to injury than in those protected by the 

 shell. The results show that although the first three legs are 

 often lost by the animals, yet they do not regenerate more 

 readily than do the other appendages protected by the shell, and 

 some of which at least can scarcely ever be lost or injured. 



The question may be asked : Do we find regeneration more 

 common in animals and plants that are most exposed to danger .'* 

 If, as several naturalists believe, this is true, what explanation 

 may be offered .'' The most frequent " explanation " is that in 

 some way this is the result of natural selection, but it may be also 

 "explained" in another way ; namely, only those animals that had 

 from the beginning the power of complete regeneration of a lost 

 part can exist where the danger exists. In other words, only 

 these forms can extend their area of distribution over places 

 dangerous to their existence. At present, however, we lack 

 the data to show that regeneration is really more frequent in 

 animals and plants that are most exposed. 



V. 



In both unicellular and multicellular forms it has been found 

 that below a certain limit of size a part fails to regenerate, 

 although in some cases the small piece may live to a time when 

 larger pieces have begun or completed their regeneration. The 

 same result has been obtained for nucleated fragments of the 

 unsegmented ^%^. Do' we meet here with the same problem 

 mentioned above, where a leg does not give rise to a newt, 

 although a newt gives rise to a leg, or are the two problems 

 quite different } I am inclined to think that they are different, 

 because the small pieces may come from regions of the body 

 that are capable of regeneration, and if the small pieces are 



