378 THE ZOOLOGIST. 



the Waxwing, A7tipelis garrula, the Pastor Starling, Pastor roseus, 

 and Pallas's Sand Grouse, after an interval sometimes of many- 

 years, or sometimes for two or three years in succession. The 

 Waxwing will overspread Western Europe in winter for a short 

 time. It appears to be equally inconstant in its choice of summer 

 quarters, as was shown by Mr. J. Wolley in Lapland. The Eose 

 Pastor regularly winters in India, but never remains to breed. 

 For this purpose the whole race seems to collect and travel north- 

 west, but rarely, or after intervals of many years, returns to the 

 same quarters. Verona, Broussa, Smyrna, Odessa, the Dobrudscha 

 have all during the last half-century been visited for one summer 

 by tens of thousands, who are attracted by the visitations of 

 locusts, on which they feed, rear their young, and go. These 

 irruptions, however, cannot be classed under the laws of ordinary 

 migration. Not less inexplicable are such migrations as those of 

 the African Darter, which, though never yet observed to the north 

 of the African lakes, contrives to pass, every spring, unobserved 

 to the lake of Antioch in North Syria, where I found a large 

 colony rearing their young, and which, so soon as their progeny 

 was able to fly, disappeared to the south-east as suddenly as they 

 had arrived. 



There is one possible explanation of " the sense of direction 

 unconsciously exercised," which I submit as a working hypothesis. 

 We are all aware of the instinct, strong both in mammals and 

 birds without exception, which attracts them to the place of their 

 nativity. When the increasing cold of the northern regions, in 

 which they all had their origin, drove the mammals southward, 

 they could not retrace their steps, because the increasing polar 

 sea, as the arctic continent sank, barred the way. The birds 

 reluctantly left their homes as winter came on, and followed the 

 supply of food. But as the season in their new residence became 

 hotter in summer, they instinctively returned to their birth- 

 places, and there reared their young, retiring with them when the 

 recurring winter impelled them to seek a warmer climate. Those 

 species which, unfitted for a greater amount of heat by their more 

 protracted sojourn in the northern regions, persisted in revisiting 

 their ancestral homes, or getting as near to them as they could, 

 retained a capacity for enjoying a temperate climate, which, very 

 gradually, was lost by the species which settled down more 

 permanently in their new quarters, and thus a law of migration 



