4 1 6 Studies in tlie Theory of Descent. 



Variability of this kind might well be designated 

 as secondary, in contradistinction to primary varia- 

 bility ; the latter (primary) depends upon an un- 

 equal reaction of the individual organisms to 

 external influences, the former (secondary) results 

 from the unequal strength and rate of the action of 

 the innate laws of growth governing the organism. 

 In both cases alike exceeding variability may 

 occur, but the causes producing this variability are 

 dissimilar. 



The different stages of larval development would 

 thus frequently display independent variability in a 

 manner similar to the pupal or imaginal stages, 

 since they can show individual variability while the 

 other stages of development remain constant. This 

 appearance of independent variability in the dif- 

 ferent stages of the larval development, however, 

 is in truth deceptive we have here in fact a kind 

 of wave of variability, which passes downwards 

 through the developmental stages, becoming 

 gradually weaker, and finally dying out com- 

 pletely. 



In accordance with this, we very frequently find 

 that only the last or two last stages are variable, 

 while the younger stages are constant. Thus in 

 Macroglossa Stellatarum, the larvse are constant in 

 the first, second, and third stages, but become vari- 

 able in the fourth, and in the fifth stage first show 

 that high degree of variability which has already 

 been described in detail (See. PI. III., Figs. 3 1 2). 



