Regeneration in Larval Legs of Silkworms. 599 



organ." It is not a sting nor in any way an effective weapon 

 of defense, as even where long and conspicuous (1^ in. long) it 

 is weak and easily bent. Nor does it secrete an acrid or ill- 

 smelling fluid. Certainly in the silkworm it has had for many 

 hundreds of generations no possible function as a weapon. It 

 is interesting to note that this useless organ is not regenerated. 



RelatiojT. of Regeneration to Natural Selection. 



This suggests to us a consideration of the relation of regenera- 

 tion, as we have observed it in the silkworm, to its causes, or at 

 least to natural selection as an explaining cause. If the caudal 

 horn is now a useless organ in the silkworm body its lack of 

 capacity to regenerate (loss of capacity, if it ever had it) would 

 seem to favor the theory of the natural selectionists concerning 

 regeneration. At first glance, also, the retaining of the regenera- 

 tive capacity of the legs, useful organs, may seem to favor this 

 theory. But it must be borne in mind that the silkworm has been 

 for approximately 5000 years a domesticated animal cared for 

 under such conditions as to make the natural loss of legs almost an 

 impossible occurrence. 



Perfectly protected against such natural enemies as bite ofi^ 

 legs, there has certainly been nothing of that sharp necessity, 

 during all the life of countless successive generations of silkworms, 

 which is supposed to be the basis for maintaining the advan- 

 tageous capacity for regeneration. There has been a clear field 

 for panmixia. But the regenerative capacity still exists in effec- 

 tive degree. The silkworm offers little aid and comfort to those 

 who would explain regeneration wholly as a phenomenon fostered 

 and maintained by natural selection on a basis of utility. 



