THE QLACIAL EPOCH. 



29 



Prfe-Plio- 

 cene Glacial 



during which it seems to me that many of the present species of Charadriidae were Post -? i llo ~. 

 differentiated, the Post-Pliocene Glacial Epoch, which, according to Croll, occurred during Epoch, 

 the last period of great eccentricity of the earth's orbit. Speaking in round figures, we 

 may say that it is 200 thousand years since the Post-Pliocene Glacial Epoch began, and 

 that it lasted rather more than 100 thousand years, so that it is rather less than 100 

 thousand years since it came to an end. 



The Glacial Epoch which differentiated the genera of Charadriidse may have been 

 Miocene or even Eocene, but if the palseontologic evidence be of any value, it must at least Epoch. 

 have been Prae-Pliocene ; and it is not beyond the bounds of possibility that this Pree-Pliocene 

 Glacial Epoch was caused by the period of great eccentricity of the earth's orbit, which 

 occurred from 800 to 900 thousand years ago. 



It would, however, be a great mistake to attach any importance to these dates, which Fact hnpor- 

 may after all be wildly wrong. The point to which I desire to call especial attention is the date'unim- 

 nature and sequence of those great events, which appear to me to have left indelible traces P ortant ' 

 upon the existing species of Charadriidge, which no one can fail to see who carefully studies 

 the geographical distribution of this interesting family. 



Accepting these data as all the evidence that we can at present obtain on the subject, 

 we may proceed to fill in the details of this hypothetical history of the family Charadriidse. 



Some time before the Pliocene Age, the ancestors of the two hundred species, which 

 are now classed in the family of Charadriidae, consisted of only one species, which lived 

 during nine months of the year on the shores of the Polar Basin. How long it had lived 

 there, or whence it came, it is impossible to say. It may have emigrated to the 

 land of the midnight sun any time before the Prae-Pliocene Glacial Epoch, which 

 must have banished all bird-life from the Arctic Regions. At any rate it had been 

 there long enough to have become circumpolar, and possibly long enough to have 

 acquired the habit of feeding by night, as well as by day, so as to reduce the risk of long 

 migrations in search of light, during the three months' winter darkness, to a minimum. 

 The wandering habits, acquired by the necessity of making a short trip southwards every 

 winter, and the old custom, so conspicuous amongst most birds, of driving away their young 

 to find breeding-grounds at a distance from their parents, no doubt kept them well mixed 

 together ; so that modification was slow, and whatever change gradually took place, was 

 shared by the whole species, being continually distributed by constant interbreeding. The 

 comparatively small area of the Polar Basin, with its archipelagos of islands, offered every 

 facility for constant intermarriages between individuals from various districts, who crossed 

 and recrossed the Arctic Ocean at will in search of an unoccupied breeding-ground or an 

 unmarried mate. The conditions of life remained much the same year after year, and 

 neither isolation, nor its usual concomitant differentiation, took place. But the coming on 

 of a glacial period changed all this. 



Ice began to form at the North Pole : perpetual snow covered the mountain-ranges of 

 Greenland : glaciers crept down to the sea ; and in spite of the short hot summer, the 



Original 

 home of the 

 ancestral 

 species. 



The species 

 kept homo- 

 geneous by 

 inter- 

 breeding. 



