The Influence of External Conditions 15 



to temperatures higher or lower than those to which they are 

 normally subjected. If the change of temperature is gradual, 

 the organism may become adapted to a temperature that would 

 have been fatal if met at once. 



Somewhat similar results have been found by subjecting ma- 

 rine animals to water containing less salt. If the change is 

 gradual, the animal will become adapted to the new density. 

 The extent to which the process may be carried differs greatly 

 in different species. In some cases the animal may be gradually 

 transferred even to fresh water. There are also other animals 

 that may pass at once from salt to fresh water without serious 

 injury. Salmon and shad leave the ocean to migrate up the 

 rivers, and other fish do the same thing. Conversely, the 

 young salmon migrates back to the sea. The number of fish 

 that will stand as great a change as this is, however, limited, 

 although many oceanic species will live in water much less salt 

 than that of the sea. 



NON- ADAPTIVE RESPONSES 



In contrast to the few cases of adaptive structural responses 

 to the environment there are quite a number of cases in which 

 definite structural responses occur that are not adaptive. One 

 of the interesting points connected with these responses is that 

 the differences effected by changes in the environment have been 

 shown in some cases to resemble the kind of differences that 

 separate species from each other; but whether species have 

 really originated, either directly or indirectly, in this way, must 

 be carefully considered later. 



Influence of Temperature on the Coloration of Butterflies 



The earliest experimenters on the influence of temperature 

 on butterflies were Dorfmeister (1864) and Weismann (1875). 

 Earlier naturalists, who were familiar with seasonal dimor- 

 phism in butterflies, had supposed, it is true, that differences in 

 temperature might be responsible for the differences in color 

 that characterize the summer and the winter broods; but it 



