154 Traces of Unity in the 



great effort they are about to make, not only by taking 

 long flights together, but also by roosting with them on 

 oziers and other water-loving trees, and so teaching 

 them to forget their nests. One day all are there : the 

 next all are gone. An irresistible impulse has impelled 

 them to go together, and not to rest until they reach 

 the north of Africa. They cannot stay : they have 

 little liberty in the choice of the course they have to 

 take : and, where a wide expanse of sea has to be 

 crossed, not a few, it is believed, are drowned, unless 

 there happen to be within reach ships upon the rigging 

 of which they may rest awhile. In due time also an 

 equally irresistible impulse drives them back by the same 

 route to their old haunts in the north, and great is 

 their distress if, on arriving there, the well-remembered 

 nests are not to be found. Year by year so it happens : 

 as the plants begin to fade, and the insects to disappear, 

 the birds depart : as the plants recover their leaves and 

 flowers, and insects reappear upon the scene, the birds 

 return. It seems to be a matter of circumstance in 

 which the birds are as passive as the insects, almost 

 as passive as the plants a matter in which the vital 

 movements have to do, not with automatism, but 

 with periodic changes in the position of the earth in 

 relation to the sun, and with other cosmical changes. 



So too with the salmon. The eggs of this noble 

 fish are deposited in a trench scooped out by one or both 

 the parents in the gravelly bed of a rapid and rushing 

 stream, and there left, ten days or thereabouts being 

 spent in the process of oviposition and fertilization. 

 Somewhat later the parents, then miserable, lank, scarcely 

 eatable kelts, no longer caring for their eggs, betake 

 themselves to the sea, and nothing further is known of 

 them until, after a period varying from six weeks to 



