On the Seasonal Dimorphism of Butterflies. 1 1 5 



me to depend upon unlike external influences, 

 the inherited constitution itself being dissimi- 

 lar because the individuals have been at all 

 times exposed to somewhat varying external 

 influences. 



A change arising from purely internal causes 

 seems to me above all quite untenable, because I 

 cannot imagine how the same material substratum 

 of physical constitution of a species can be trans- 

 ferred to the succeeding generation as two op- 

 posing tendencies. Yet this must be the case if the 

 direction of development transferred by heredity 

 is to be regarded as the ultimate ground both of 

 the similarity and dissimilarity to the ancestors. 

 All changes, from the least to the greatest, appear 

 to me to depend ultimately only on external in- 

 fluences ; they are the response of the organism to 

 external inciting causes. It is evident that this 

 response must be different when a physical consti- 

 tution of a different nature is affected by the same 

 inciting cause, and upon this, according to my 

 view, depends the great importance of these con- 

 stitutional differences. 



If, under " heredity," we comprise the totality of 

 inheritance that is to say, the physical constitu- 

 tion of a species at any time, and therefore the 

 restricted and, in the foregoing sense, pre-de- 

 termined power of variation, whilst under " adap- 

 tation" we comprehend the direct and indirect 

 response of this physical constitution to the changes 



I 2 



