IO4 Studies in the Theory of Descent. 



terflies, as I believe I have proved, temperature 

 plays the chief part, and as this only oscillates be- 

 tween rather narrow limits, it admits of no great 

 differences of coloration. 



The question thus suggests itself, whether 

 species of butterflies only oscillate between two 

 forms, or whether climatic change, when sufficiently 

 great to produce variation, does not again origi- 

 nate a new form. Inasmuch as the reversion 

 experiments with seasonally dimorphic butterflies 

 appear to correspond with the latter view, I 

 believe that this must be admitted. I am of 

 opinion that an old form never again arises 

 through change of climate, but always a new 

 one ; so that a periodically recurring change of 

 climate is alone sufficient, in the course of a long 

 period of time, to admit of new species arising 

 from one another. This, at least, may be the case 

 with butterflies. 



My views rest essentially upon theoretical con- 

 siderations. It has already been insisted upon, 

 as results immediately from the experiments, that 

 temperature does not act on the physical constitu- 

 tion of the individual in the same manner as acid 

 or alkali upon litmus paper, i. e., that one and the 

 same individual does not produce this or that 

 coloration and marking according as it is exposed 

 to warmth or cold ; but rather that climate, when 

 it influences in a similar manner many succeeding 

 generations, gradually produces such a change in 



