Having endeavoured to ascertain the manner in which this change of colour takes place, we arc now ready to in vestigate the causes by which it is produced. As this change of colour in winter is peculiar to the animals which inhabit cold countries, we may safely conclude, that tem perature exercises over it a powerful influence. This sup position is countenanced by the slowness of the process of change of colour in a mild autumn, and its imperfect ac complishment 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 not thwards, until at Greenland the bird is of a pure white. If this change of colour proceeds from a renewal of feathers, bete at least the colour of the feathers must be considered influenced by the temperature, and consequently a corresponding influence must be exer cised on the secreting organs.
The distribution of colour in the animal kingdom in ge neral seems to follow the same law ; the deep and bright colours prevailing in the warm regions, while the tints of the colder regions are pale and dull. Are we to conclude, that cold diminishes the action of the vessels which furnish the colouring matter, and, when intense, entirely suspends their functions ? or are we to consider light as in part con cerned in producing the effect ? Iu general, the fur of qua druptds, and the feathers of birds, are darkest where ex posed to the light, and are pale coloured towards the base ; but in the instances before us this difference disappears, and a complete uniformity in all the parts of the covering prevails. Besides, the change does not take place on all parts of the body at the same time, but appears in spots, or on single hairs of feathers. Light therefore has little in fluence.
There is another agent besides cold apparently concern ed in the change in the colour of the feathers of birds. In all the feathers become more vivid in spring, and certain spots appear which are not observable at other seasons. This brightness of plumage and these spots con tinue only during the season of love ; and hence, instead of supposing them the production of new moultings, we con sider them as resulting from the action of the generative impulse on the colouring secretions.
In attempting to account for these phenomena of nature, it has generally been supposed, that these periodical changes of colour take place, to enable the animals more readily to escape from their prey during the winter season.
Thus Montagu, in reference to this subject, has the fol lowing reflections: ac Here we perceive the ptarmigan in variably effect this curious, and, we may add, most pro vidential change ; for if the young of those birds at first assumed their snowy winter plumage, while yet the sur face of the ground was not consonant with their colour, few would escape the piercing eye of the falcon or the eagle, in the lofty and exposed situations they are found to inhabit." To suppose that in winter the ptarinigan is rendered white, to cause it to resemble the snow and de ceive birds of prey, and that the alpine hare undergoes similar changes for the same purpose, would be to yield our assent to public opinion. But all our conclusions con cerning final causes ought to be the result of very extend ed observations ; and if our observations on this subject are extended, some difficulties will present themselves. if this white colour yields protection to the ptarmigan and alpine hare, it must enable the ermine, an animal well qua lifted to provide for its wants at all times, by its determin ed boldness, extreme agility, and exquisite smell, to prey with greater certainty upon its defenceless neighbours. What protection, we would ask, is afforded to the black guillemot, (luting the winter, by its mottled plumage, or to the little auk by its white chin, since tic whiter they be come, so much the more unlike the dark colour of the water ? Protection from foes, therefore, cannot be con sidered as the object of nature in these curious changes, especially as the change of colour, always from dark to white, does not differ, however different the habits and even stations of the animals may be.
Perhaps the laws of chemistry may furnish us with a more consistent and plausible explanation. If the radiat ing power of bodies with regard to heat be inversely as their reflecting power, a conclusion very generally admit ted, then the white winter dress of these animals must be better calculated for retaining the heat generated in their bodies by the vital principle, than any other coloured dress which would possess greater radiating power, and, con sequently, would more readily contribute to the reduction of their temperature. It is probable, therefore, that these changes in the quantity and colour of the clothing of ani mals, are designed by nature to regulate their temperature in the different seasons of the year.