On the Seasonal Dimorphism of Butterflies. 65 



influences which act only on certain stages in the 

 development of the individual, even when the 

 action is cumulative and not sudden, only affect 

 those particular stages without having any effect 

 on the earlier or later stages. This law is 

 obviously of the greatest importance to the com- 

 prehension of metamorphosis. Lubbock 1 has 

 briefly shown in a very clear manner how the 

 existence of metamorphosis in insects can be 

 explained by the indirect action of varying con- 

 ditions on the different life-stages of a species. 

 Thus the mandibles of a caterpillar are, by adap- 

 tation to another mode of nourishment, exchanged 

 at a later period of life for a suctorial organ. Such 

 adaptation of the various development-stages of a 

 species to the different conditions of life would 

 never give rise to metamorphosis, if the law of 

 homochronic, or periodic, heredity did not cause 

 the characters gradually acquired at a given stage 

 to be transferred to the same stage of the follow- 

 ing generation. 



The origin of seasonal dimorphism depends 

 upon a very similar law, or rather form, of inheri- 

 tance, which differs from that above considered 

 only in the fact that, instead of the ontogenetic 

 stages, a whole series of generations is influenced. 

 This form of inheritance may be formulated some- 

 what as follows : When dissimilar conditions 



1 " On the Origin and Metamorphoses of Insects," London, 



18714. 



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