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 oj 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 
