December 11, 1903.] 



SCIENCE. 



743 



wheu the larvsB were about nineteen days 

 old, we found the first lot very uniform and 

 the second very unequal in size, the differ- 

 ence between the heaviest and lightest in 

 the first lot being 19 mg. as against 45 nig. 

 in the second. To the second lot belonged 

 the smallest individual among the season's 

 entire generation of worms, while, on the 

 other hand, this competitive lot boasted one 

 precocious individual weighing more than 

 the average among other lots of well-fed 

 worms, holding, indeed, third place among 

 the 'heavj' weights' of the season. 



The records of size, when these larvte had 

 finished feeding and were ready to spin, 

 show the final results of the competitive 

 and non-competitive life of the under-fed 

 lots: in the first lot, with the individuals 

 isolated, the difference in weight between 

 the largest and smallest was 229 mg., in 

 length 8 mm. ; in the second, with the indi- 

 viduals together and competing, the differ- 

 ence in weight was 901 mg., in length 22 

 mm. These figures speak for themselves 

 and offer a pretty illustration of the non- 

 combative but equally strenuous struggle 

 for existence which occurs when an in- 

 adequate food supply results in a struggle 

 between closely allied and hence competing 

 forms, to the prosperity of some and the 

 decline of other members of the species 

 thus divided against itself. 



A second reason for isolating the larva? 

 individually is that individual records ex- 

 tending over long periods of time are not 

 otherwise possible. The data consist of 

 individual records concerning Characters, 

 in part enumerated below, of 630 indi- 

 viduals of Bombyx mori belonging to thi-ee 

 generations (1901-03). 



The studied characters which are per- 

 tinent to the present discussion may be 

 listed as follows: 



1. Those relating to size as indicated l)y 

 weight of larva, of the cocooned pui>a, of 



the cocoon or pupa alone and of the adult 

 upon emergence. 



2. Those relating to the prompt perform- 

 ance and normal occurrence of physiolog- 

 ical functions such as the moulting and 

 spinning of the larva and the emergence 

 of the adult. 



3. Fertility in so far as it is indicated 

 by the number of eggs laid. 



4. Mortality among the variously nour- 

 ished lots as indicated by the death rate. 



Some generalizations already reached 

 through the study of the data may be 

 briefly summarized as follows: 



1. As to the substitution of lettuce for 

 mulberry as silkworm food, the experiment 

 has been tried only witli the generation of 

 1903, and that on a rather small scale. 

 The 'worms' have adapted themselves to 

 this change of diet to the extent of living 

 successful individual lives and of produc- 

 ing eggs which bear all the earmarks of 

 fertility, that is, have gone through the nor- 

 mal change of color from yellow to gray, 

 the result of beginning development. The 

 eggs will not hatch until March, 1904. The 

 young larviT" adopted the unusual diet very 

 reluctantly, but in later life these same 

 larvct, 'educated' to its use, ate lettuce 

 with a relish which would rival that dis- 

 played by the normal larva with its mul- 

 berry leaf. 



The most striking variation induced by 

 this lettuce regimen was that the time con- 

 sumed by the inetamorphosis was double 

 the time appointed for that of the normal 

 mulberry-fed larva— being three months as 

 compared with six weeks for the latter. 

 In the commercial world this fact would 

 offset the advantage of the lettuce, as a 

 cheaper food and as one available at all 

 seasons, by demanding twice the labor that 

 is required to rear to spinning time larvaj 

 fed on mulberry. Thus it appeai-s that the 

 lettuce experiment can not be of economic 

 value to sericulture unless it should prove 



