THE MIGRATIONS OF BIRDS 29 



rest. In one or at most two flights at night is the 

 journey done. Flying low down over the earth and 

 sea if the night be dark, filling the air with their strange 

 migratory cries, they are carried on through all weathers 

 by an instinct that only errs, moth-like, at light in the 

 gloom ; or if the night be clear, high up, often out of 

 sight, where the cold and lightness of the air would 

 numb and overcome us, they take their way and 

 arrive at the haven where they would be. 



But even before summer is over the return move- 

 ment southward begins. First the unmated birds 

 slip away. Then the birds of the year, perhaps under 

 pressure of circumstances, yield to this new migratory 

 impulse, that leads them to forswear the county and 

 country of their birth and to find their way unaided 

 across water to lands not their own. Lastly come the 

 parents, flock by flock, not in the solid phalanx that 

 they presented as a northward-going host, but silently 

 and in detachments, with the dress of spring worn out 

 by the summer's housekeeping or replaced by a new 

 but duller garb. More leisurely now and by less direct 

 routes they make their way to the Mediterranean, to 

 the iFgean, and to southward-lying lands. 



Modes of Animal Movement. — If we imagine a man 

 in a boat provided with oars and boathook we can 

 think of four different ways in which he may effect 

 movement : by punting with one of the oars against 

 the bottom of the water ; by hauling against brush- 

 wood or other obstacles on the banks ; by sculling from 

 the stern with a single oar, or by rowing with a pair 



