276 With Rod and Gun in New Enlgand 



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I do not mean to say, however, that I was bright enough to pick up 

 the clew given me so repeatedly by old hens. No ; although I have seen 

 hens, as well as other birds, driving their offspring from them many times, 

 it did not occur to me that this was the end of the thread that leads 

 through the labyrinth of migration. I came at it in another way, and only 

 followed out the thread which led to the hen and her chickens from the 

 inside, after the matter had become plain to me. 



I had been working upon a problem concerning two hypothetical 

 species of birds, and the study which this involved led me to look into the 

 origin of certain groups of birds. It then occurred to me that migration 

 of birds would greatly aid us in determining the origin of groups of birds ; 

 when, like a flash, came the thought, — why, the origin of groups of birds 

 surely furnishes a certain clew to the origin of that instinct which we 

 call the instinct of migration. I, for one, had been always thinking about 

 migration from the wrong end. Then it was that I saw that the hen and 

 other birds had been offering me the clew so long, but I, neglecting an 

 easy way in, had fairly stumbled over the wall. 



First, I must go back to the beginning ; yes, even to the beginning of 

 migration, to a time when we can be reasonably certain that there was no 

 annual migration, or at least no migration north and south, north of the 

 equator. This was during the last glacial period. 



Whether prior to this time, when the northern hemisphere was over- 

 whelmed by the great ice sheet, the fauna inhabiting the Arctic and Tem- 

 perate zones were migratory, is exceedingly difficult to say. Of one thing 

 we can be certain, and that is that all animal life either had to retreat be- 

 fore the ice sheet or be overwhelmed by it. Probably most birds retreated 

 before it. 



At a time, then, when the ice had reached its nearest point to the south 

 pole (for it is probable that there were alternate glacial periods, the ice 

 sheet covering first one hemisphere of the earth and then the other, very 

 long periods of time intervening between the two changes), the birds and 

 other animals would be forced into comparatively narrow limits. Possibly, 

 and even probably, the fauna, consisting of all classes of animals, was quite 

 limited at this time ; but then, as now, the parent birds were driving their 

 young away from them, and were causing them to spread over every possi- 

 ble space of land, east and west. But at the north they were hemmed in 

 by the ice and so could not go far in that direction. After a time, how 

 long a time we do not know, the great cold cycle began to abate and the 

 ice sheet began to melt, backing toward the north pole, slowly at first, leav- 

 ing a desolate land exposed. But Nature always hastens to clothe desolate 

 spots with verdure ; and following vegetation came insects, spreading gradu- 

 ally to the northward. The birds, induced to scatter by their parents,. 



