'Feathers ofBirds^ indef enderd of Mmiltlng, 275 
2. That these changes of colour are regulated by the temper 
rature of the atmosphere, the dark deep tints occurring during 
the heat of summer, the light-coloured ones in the cold of win- 
ter. This supposition is countenanced by the slowness of the 
Jirocess of change of colour in a mild autumn, and its imperfect 
accomplishment during a mild winter. Besides, in some ani- 
mals, such as the black guillemot, the change is never complete 
in the more temperate regions, but becomes more perfect as we 
proceed northwards, until at Greenland the bird is of a pure 
white.” In attending, therefore, to the plumage of birds in re- 
ference to this subject, the mildness or severity of the season 
should likewise be considered. By attending to this law, the 
ornithologists may detect many spurious species in the modern 
lists of British birds, even after the successful labours of Mon- 
tagu to elucidate the subject. 
3. That these changes assist in regulating the temperature of 
animals in the different seasons of the year. It is generally 
supposed, that many animals bec6me white in winter, in order 
to enable them to escape the observation of their foes, being 
nearly of the same colour as the snow. In reasoning concern- 
ing final causes, we should be careful to draw our conclusions 
from a minute and impartial view of all the circumstances of the 
case, lest we attempt to establish laws, whose operation would 
produce pain or ruin, where we intended to unfold the marks 
of wisdom and benevolence. It is obvious in this case, that 
while the ptarmigan and alpine hare would be protected from 
danger, the eagle, the cat. and the fox, would be in danger of 
starvation and death. But the fallacy of this popular opinion 
may be demonstrated from the circumstance, that the defence- 
less and the rapacious, the terrestrial and the aquatic animals, 
are equally subject to the change ; and that the order of succes- 
sion in the shades of colour is always the same, however diffe- 
rent the habits or stations of the animals in which it is observed. 
The following explanation was offered of these curious pheno- 
mena of the animal economy : “ If the radiating power of bo- 
at Sunderland. In a letter which I received from him about eight months ago, he 
stated, that he had received another specimen from the Rev. Dr Stuart of Luss, 
^yhich had been shot upon Loch Lomond- 
