36 THE INFLUENCE OF ENVIRONMENT ON 



rather smaller, and has more black at the bases 

 and less at the tips of the wings than the other 

 variety. The green veining on the under surface 

 is also more marked. If the pupae of the summer 

 brood are placed in a refrigerator for three months, 

 and then in a hothouse, they hatch in about three 

 weeks as the winter form. 



These illustrations I have cited show the direct 

 action of cold producing a result which can be 

 calculated on beforehand with certainty. By the 

 long-continued application of heat the summer form 

 has been produced, and if the environment is altered 

 and cold substituted for heat, we reverse the action 

 and bring back the summer form to the original 

 winter one. 



Now let me refer to an instance in which the 

 evidence is as yet incomplete an instance some- 

 what more technical than the previous ones. I 

 choose it purposely because it is of a somewhat 

 abstruse kind, and will therefore serve to show 

 that some of the more intricate problems con- 

 cerning the exact nature of the relation between 

 environment and structure and habits may be well 

 within our grasp. 



Many animals possess the power of changing 

 colours e.g., the chameleon, frog, many fish, and 

 the cuttle-fish. This power depends on the 

 presence of chromatophores or pigmented bodies 

 found in the skin, which have the power of 

 changing shape. These are of different colours 

 and are arranged in layers, some near the surface, 

 some deeper; the light yellow cells being most 



