DARWINISM TO-DAY. 



berry silkworm carried on by R. G. Bell and myself. (For detailed ac- 

 count of this work see Science, N. S., Vol. XVIII, pp. 741-748, 1903.) 



"One of the races of the mulberry silkworm, Bombyx mori, has 

 been the subject of experiments directed toward a determination of 

 the exact quantitative relation which quantity and quality of food 

 bear to the development and variations of the individual insect and 

 its progeny. . . . 



"The insect, Bombyx mori, has a complete metamorphosis, tak- 

 ing no food as an adult, so that the experimental control of the 

 Experiments feeding has been necessary only during the larval or 

 with silkworms, 'silkworm' stage. The larval life is subdivided into 

 five stages clearly set off from one another by the intervening 

 moults, of which there are normally four, and these substages have 

 been useful when an alteration of food conditions during a sharply de- 

 fined shorter time than the entire larval life was desirable. . . . The 

 change in quantity of food has consisted in altering the amount of 

 mulberry leaf served to the larvae, the control of which has bee.n 

 secured as follows : It has been determined through experience with 

 normal larvae that each will consume a certain amount of food in 

 a certain number of hours (increasing in amount with the increas- 

 ing age and size of the larva), this amount representing the 

 optimum amount of food for the normal individual and necessitating 

 as many daily meals as are required to keep any but the moulting 

 larva constantly supplied with fresh food. This amount determined, 

 a tolerably definite small proportion of the optimum amount has 

 been allotted the individuals which were sentenced to short rations, 

 which, roughly speaking, might be listed as one-quarter the optimum 

 amount during earlier stages and one-eighth during the late larval 

 .stages. This one-fourth, one-eighth, or whatever it may have been 

 numerically, was, at any rate, as small an amount of food as was 

 compatible with mere life. . . . 



"These experiments have extended over a period of three years, 

 covering as many generations of the insect. The data gathered 

 (being the measurements, weight, and duration of each larva in each 

 of its five states; the time of spinning, weight of silk and weight 

 and duration of each pupa ; and the weight, size, pattern, and 

 fertility of female of each imago) furnish material, then, for a 

 study of the effects of under-feeding upon individuals during a 

 single generation (the 1903 generation or that of 1902 or 1901), 

 during two successive generations (1901-02 or 1902-03), and two 

 alternating generations (1901-1903) and during three generations 

 (1901-03), a control lot having been carried for each experimental 

 lot so that what is modified may confidently be distinguished from 

 what is normal. . . . 



