APPENDIX 83 



individuals (and tolerably vigorous larvae they were) alive in the lot which had 

 experienced two years of famine, although every individual of the 149 hatched 

 was carefully preserved and royally fed a fact which goes to prove that the 

 equipment at birth of many of these larvae was inadequate. 



The fact that some larvae of starved ancestry have exhibited a superiority 

 over their fellows, in surviving and recovering from hard conditions, is testi- 

 mony for the existence of individual variations which can not be defined anatomi- 

 cally, and yet which serve as "handles" for natural selective agents. Such varia- 

 tions might be called physiological variations, since it seems that the surviving 

 larvae must be those which are in best trim physiologically. These larvae are 

 able to make the most of the food offered to them. If competition were allowed, 

 they would probably be the individuals which would cover the area most rapidly, 

 securing whatever food there might be. But under our experimental conditions 

 there was no competition allowed and yet certain precocious individuals made 

 more grams of flesh and more yards of silk, than other larvae furnished with the 

 same amount of raw material under like conditions; that this was due to the 

 possession by the former of certain congenital qualities of adaptability can scarcely 

 be doubted. 



As to the fertility of the variously fed lots; in so far as number of eggs 

 produced is a measure of fertility, our records already demonstrate the fact that 

 the better nourished are the more fertile. Furthermore, the economy in this 

 matter practised by the starvelings is not merely numerical, quality as well as 

 quantity of eggs being affected. In witness of this point may be recalled the 

 story of the dying 1903 generation, produced from eggs of the starvelings of 

 1901 and 1902, which would seem to offer conclusive evidence that a famine 

 suffered by the parents works its way into the germ cells so that most of their 

 progeny have but a poor birthright. 



Regeneration in Larval Legs of Silkworms, in Jour. Exper. Zool., v. 1, 

 pp 593-599, 10 figs., Dec., 1904. 



Experimenters in regeneration in insects have too often overlooked the fact 

 that the imaginal (adult) legs of insects of complete metamorphosis are produced 

 not by a direct transformation of the corresponding larval thoracic legs but from 

 new centers called imaginal discs or histoblasts. These histoblasts are developed 

 from an invagination of the larval cellular skin layer (hypoderm) and only in 

 comparatively late larval life do the new developing imaginal legs lie within the 

 larval ones. It follows from this that if a larval leg be cut off in early larval 

 life the imaginal leg is in no way mutilated, and that if it appears of full size 

 and normal character in the adult insect, this is not due to restorative regenera- 

 tion but simply to its normal growth and development. If a leg be cut off in 

 late larval life, the developing imaginal leg may or may not be at the same time 

 mutilated. If mutilated, however, it will always be by a removal of much less 

 of its extent than of the extent of the larval leg taken off. A cut which severs 

 the larval leg near its base (for example, through the base of the femur), will 

 not take off more than the tarsus or perhaps part of the tibia and tarsus of the 

 imaginal leg, which, in its development, is beginning to extend into the larval 

 one. Thus if the imaginal leg be found, when the imago issues, to lack a tarsus 



