112 DARWINISM AND THE PROBLEMS OF LIFE 



could only be where there was means of sustenance, 

 the line of migration must pass over these localities, 

 and so follow the ancient line of expansion. 



This is seen in the paths of all migratory birds. 

 River-birds travel along the rivers, though also over 

 high mountains, as here also there are streams and 

 lakes that afford them sustenance ; while marsh - birds 

 go round the marshless hills. Land-birds go straight 

 ahead over the country, and only halt at the sea, and 

 diverge along its shore. The fact that European birds 

 still cross water, namely, the Mediterranean, is ex- 

 plained by the circumstance that there was not always 

 water where the waves roll to-day. In the earlier 

 periods of geology there were bridges of land from 

 Africa to Eur6pe, through Malta and Sicily as well as 

 at Gibraltar. Over these the birds could advance 

 gradually towards the north, and as a fact we find that 

 the sea is only crossed by them at these points. 



Let us now picture the migration to ourselves once 

 more. A pair of birds travel to the north, rear their 

 young there, and return with these in the autumn to 

 the old home. In the spring they all return to the 

 nesting-place of the previous year, but some of the 

 young go still further north to build their nest. The 

 journey in autumn will consist of two parts for these. 

 They have already done the larger section of it twice 

 with their parents ; the second is the stretch from the 

 nest in which they themselves were reared to the new 

 home that they have made. But both sections of the 

 journey are only the first part of the journey for their 

 offspring, as these will add to it by advancing further 



