774 NATURAL SCIENCE. d^c.. 



Young birds migrate in the autumn unattended by the old, and 

 without any previous knowledge of the route. The force, stimulus^ 

 or impelling power is what we call instinct, which may be defined as 

 a special adaptation to a special circumstance, a non-intelligent 

 action, that is instinctive, and, as it has been recently expressed,^ 

 performed in virtue of the innate and co-ordinated mechanisms of the 

 organism, an innate capacity developed by Natural Selection. 

 Instance the selection of a butterfly of the leaf upon which she lays her 

 eggs, the only leaf, it may be, which serves her grubs, and a food of the 

 kind which she herself does not eat ; the instinct of a young pointer, 

 brought up away from the parent, drawing on game and standing the 

 first time it is taken into the field ; the instinct of the young bird 

 which, independent of all guidance and unimproved by intelligence, 

 can find its path unerringly to the winter home of its parents across 

 thousands of miles of land and water. 



Regarding the origin of these ancient and complicated movements, 

 granting that birds have ascended from reptilian ancestors, they would 

 begin to emigrate from place to place as soon as the exigencies of life 

 and their powers of aerial locomotion became developed. There is, 

 however, no necessity, when considering the origin of migration and 

 those ever- varying phenomena which now attend these movements, 

 to go back to the great unthinkable ages — the cooling down period of 

 the earth's existence, when the Arctic and Antarctic zones became 

 first capable of supporting life ; the key and explanation of the mystery 

 may be found in a comparatively recent geological period, commenc- 

 ing with the slow recession of the great ice-cap from Europe and 

 North America in Post-pliocene times. 



There is nothing in the present position of bird life in the globe, 

 which can be accounted for by the present known geographical distri- 

 bution of species ; the explanation must be sought in the unreckoned 

 ages of the past. 



Mr. Dixon dwells much on what he calls inter-polar migration,, 

 and endeavours to prove the probability of some ancestral forms of 

 the Charadriidae inhabiting, during remote ages, a south polar area. 

 He considers that long journeys now taken by some species of waders, 

 to south of the Equator — as the knot, curlew sandpiper, sanderling, 

 and grey plover — and reaching over 140 degrees of latitude, or nearly 

 10,000 miles direct, are the result of the transfer of these species from 

 the North Polar Basin to the South Polar Basin during favourable 

 intervals of chmate. Unfortunately for Mr. Dixon's theory, there is 

 absolutely no proof in the present distribution of bird life that the 

 antarctic regions were ever the breeding quarters of the progenitors 

 of the vast bird hosts which now repair in the breeding season to the 

 immense solitudes surrounding the North Pole. All scientific evidence 

 points in the opposite direction, and to the conclusion that life has 



' "The Study of Animal Life," J. A. Thcmson, chap, x., Instinct. 



