OTHER THEORIES OF SPECIES-FORMING. 301 



individuals of each single lot whose environment, in so far as food, 

 temperature, room, humidity, etc., constitute it, is identical. 



"For example, referring again to the weights at spinning time of 

 the larvae of 1903, it is true that, although each lot has a modal 

 class of weights to which the majority of its individuals belong and 

 about which the rest of the lot distributes itself rather symmetrically, 

 the extremes are surprisingly distant from one another. Thus in 

 lot i (the normal control lot) the extremes are 1,540 and 2,530 

 mg. ; in lot 2,* 800 and 1,402 mg. ; in lot 3, 1,180 and 2,170 mg. ; 

 in lot 4, 690 and 1,204 mg- ; in lot 5, 1,370 and 2,100 mg. 



"That is to say, identical feeding has not made identical full- 

 grown larvae out of individuals which undoubtedly varied congeni- 

 .tally at the start, those variations in embryo standing at birth in 

 the same relation to one another that they stand in the adults, hav- 

 ing merely been smaller and less readily discernible in early life, 

 although manifestly present in delicately measurable degree in the 

 earliest records made upon normal individuals. For example, 

 weight measurements taken immediately after the second moult, 

 range in one lot from 21 to 39 mg., or 60 per cent, of the modal 

 weight, while the weights in this same lot at spinning time, some 

 five weeks later, range from 534 to 2,080 mg., or 85 per cent, of 

 the mode for the lot. These embryonic but potentially large varia- 

 tions have simply 'grown up' along with the insect and are as truly 

 congenital in the adult as they were in the newly hatched larva. 

 This would seem to place quite conclusively in the category of 

 congenital variations some part of those variations (in size and pro- 

 portions of parts) which are commonly, and properly to some 

 degree, called acquired. 



"That conditions of alimentation bear a directive relation to func- 

 tional activity, may be demonstrated by reference to the records of 

 the physiological functions of moulting, spinning, pupating, and 

 emerging, of the individuals of the experimental lots. 



"An abnormal extension of the time needed for the metamor- 

 phosis follows upon a reduction of the food supply. The degree 

 of extension depends with the utmost nicety upon the amount of 

 food given the larvae. For example, among the 1901 generation of 

 silkworms, one control lot of twenty larvae was given the optimum 

 amount of food, a second lot of twenty larvae one-half this amount, 

 and a third lot of twenty larvae one-quarter of the amount. To 

 take the time of the fourth moulting as an illustration, the moulting 

 was begun by the first lot, which led the way by two and a half 

 -days, at the end of which the second lot began to moult, while 



* See table, next page, for the history of each lot. 



