26 Studies in the Theory of Descent. 



generations. The former always takes place very 

 slowly ; the latter may occur in a shorter time. 



With regard to the duration of time which is 

 necessary to produce a new form by the influence 

 of climate, or to transmit to a succeeding gene- 

 ration a new form already established, great 

 differences occur, according to the physical nature 

 of the species and of the individual. The expe- 

 riments with Prorsa already described show how 

 diverse are individual proclivities in this respect. 

 In Experiment No. 12 it was not possible out of 

 seventy individuals to substitute Prorsa for the 

 Levana form, even in one solitary case, or, in 

 other words, to change alternating into continuous 

 inheritance ; whilst in the corresponding experi- 

 ments of former years (Experiment 10, for 

 example), out of an equal number of pupae three 

 emerged as Prorsa, and one as Porima. We 

 might be inclined to seek for the cause of this 

 different behaviour in external influences, but we 

 should not thus arrive at an explanation of the 

 facts. We might suppose, for instance, that a 

 great deal depended upon the particular period of 

 the pupal stage at which the action of the elevated 

 temperature began whether on the first, the 

 thirtieth, or the hundredth day after pupation 

 and this conjecture is correct in so far that in the 

 two last cases warmth can have no further influence 

 than that of somewhat accelerating the emergence 

 of the butterflies, but cannot change the Levana 



