84 INHERITANCE IN SILKWORMS, I 



but to possess a complete femur and tibia, this is no indication that there has 

 been a partial regeneration; there may have been none whatever. 



To make a definite test of the capacity of the silkworm to regenerate lost 

 parts, legs, both thoracic and abdominal, were cut off of the larva at various ages 

 and at various places between the tarsus and the body, and notice was taken of 

 whether or not regeneration of these legs took place before pupation, and if so 

 in what degree, and whether normally, i. e., so as to produce an exact replica 

 of the lost leg, or not. 



The results of the experiments show, (a) that the larva of the silkworm 

 moth, Bombyx mori, has the capacity of regenerating its thoracic and abdominal 

 (prop-) legs from the stumps of these legs, but not from the body (trunk), i. e., 

 that each leg has the capacity to regenerate any distal part from any proximal 

 part, but that the body can not produce a wholly new leg; (b) that this regenera- 

 tion shows externally not after the first moulting after the mutilation but after 

 the second moulting, and that the regenerative processes are completed with the 

 appearance of the new parts after this second moulting succeeding the mutilation. 



The small caudal horn, a pointed non-segmented, but movable, process pro- 

 jecting upward from the dorsal surface of the penultimate abdominal segment 

 was cut off in many larvae (silkworms) of various ages, and in no case was 

 there the slightest regeneration. After the first moulting succeeding the mutila- 

 tion the new skin always extended smoothly over the place where the horn had 

 been, without any sign of scar. 



The function of this horn, which occurs on some other lepidopterous larvae, 

 notable and characteristically on the larvae of the Sphingid moths, is unknown. 

 It has been explained by some entomologists as an ornament, by others as a 

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

 defense, as even where long and conspicuous (*/j in. long) it is weak and easily 

 bent. Nor does it secrete an acrid or ill-smelling fluid. Certainly in the silk- 

 worm 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. 



Relation of Regeneration to Natural Selection. This suggests to us a con- 

 sideration of the relation of regeneration, as we have observed it in the silk- 

 worm, 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 regenerative 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 off 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 main- 

 taining the advantageous capacity for regeneration. There has been a clear 

 field for panamixia. But the regenerative capacity still exists in effective degree. 

 The silkworm offers little aid and comfort to those who would explain regenera- 

 tion wholly as a phenomenon fostered and maintained by natural selection on a 

 basis of utility. 



