Page:The Effect of External Influences upon Development.djvu/29

Rh to be investigated specially from this point of view before it could be regarded as one of seasonal double adaptation.

Cases of seasonal protective dimorphism in many Arctic mammals and birds have, however, long been known, and only differ from the instance just mentioned in the fact that the two kinds of adaptive colouration do not concern two successive generations, but appear successively in the same individual. In these cases, also, an external stimulus—cold—seems to decide whether the summer or the winter coat is to be developed. A complicated nerve-mechanism must here exist, which, at the stimulus of a certain temperature, affects 'that part of the nervous system which presides over the nutritive and chemical changes involved in the growth of hair and the appearance of the bubbles' of gas in it. A few careful observations have been made which prove this to be true. Take, for instance, those of Captain J. Ross on a Hudson's Bay Lemming. As long as the animal was kept in the cabin, and so shielded from the low temperature, it retained its summer coat through the winter; but after being placed on deck in a cage, and exposed to a temperature of 30° below zero, it changed colour and became almost entirely white at the end of a week.

But while seasonal adaptive dimorphism can at