The Bio7iomics of South African Insects. 453 



June 30. The weight of the No. 16 pin (D. F. Taylor's) 

 was obtained by weighing three sets of ten similar pins. 

 The weight of the first ten was •7060 grammes, of the 

 second and third "7950 grammes. The average weight of 

 a pin was therefore '07953 grammes, and this number 

 added to "06275 was deducted from each of the specimens 

 weighed on June 29 (see p. 456) : added to '063 it was 

 deducted from the specimens of the experiments recorded 

 above, and all others weighed on June 30 (see pp. 451, 

 456). On the latter date the cork foot was weighed 

 at the beginning of work, in the middle, and at the end. 

 On all three occasions it weighed "063 grammes. 



The consideration of the experiments on Scsamics is 

 better deferred until after describing those upon arcliesia, 

 although it is at once evident that no positive conclusions 

 can be drawn as to the nature of the environmental 

 stimulus. The negative character of the results obtained 

 induced Mr. Marshall finally to form the ojDinion quoted 

 below. 



" Salishiry, Feb. 26, 1902. — I do quite agree with you 

 now that in the case of Precis the evidence is sufficiently 

 strong to show that climate has ceased to operate as the 

 stimulus which calls forth the seasonal change. But I do 

 not think that this view is applicable to other genera 

 whose changes coincide closely with the changes in 

 climate. The theoretical proposition I would suggest is 

 that at its inception seasonal change was but slight and 

 then due entirely to climatic action, such cases doubtless 

 occurring at the present time. Any markedly useful 

 variations of this kind would then be preserved and 

 accentuated by natural selection, but climatic causes would 

 still remain the controlling factor. Finally, as in Precis, 

 the influence of natural selection would attain its maximum, 

 and the seasonal changes would then take place solely as 

 a result of this principle and irrespective of the influence 

 of climate. It remains to be seen whether this can be 

 proved by experiment." 



On pp. 455 to 458 it will be seen that there are still 

 hopes that the operation of some environmental stimulus 

 may yet be discovered in the case of Precis. 



The results obtained from the smaller* series of experi- 

 ments upon P. arehesia are even more negative than those 

 yielded by P. sesamns, as will be seen by a glance at the 

 Table below, giving a complete account of all that has 

 been as yet done. 



