DISPERSAL 153 



the plains to the hills of Nepal in the hot season, and return 

 in the cold season, carrying their young with them. 



Such seasonal migrations may be on any scale, ranging from 

 the thousand-mile journeys of some birds, to the crested 

 newt's spring migration to the nearest pond for the purpose 

 of laying its eggs, or the even shorter journey made by aquatic 

 worms and rotifers, when they repair to the hot surface- 

 layers of conferva in ponds for the same purpose. 



7. Amongst the various means of dispersal we can dis- 

 tinguish roughly between the more voluntary, or at any rate 

 active, migration of the animal itself on the one hand, and the 

 numerous means of *' accidental " dispersal on the other. 

 An enormous number of the smaller species of animals get 

 about from one place to another by special means of transport 

 other than their own legs, wings, or cilia. In certain cases, as 

 when the larva has a special instinct or tropism which leads 

 it to hang on to a particular animal, the dispersal may be 

 apparently almost solely for the purpose of disseminating the 

 species. Thus, the glochidia larvae of the mussel have a special 

 reaction to certain fish which causes them to shoot out the sticky 

 threads by w^hich they hang on to their temporary host. But 

 at the same time, many of these cases involve the extraction 

 of food from the host while the animal is hanging on, and so 

 the dispersal is to that extent a secondary consideration. 



Amongst the more curious means of dispersal is that 

 adopted by the Emperor penguin, which breeds in mid- 

 winter on the sea-ice on certain parts of Antarctica. Being 

 a large bird, development takes rather a long time, and in spring 

 the young birds are not always ready to take to the water by 

 the time the sea-ice begins to break up and drift north. This 

 difficulty was seen to be overcome by the birds taking their 

 chicks to the edge of the sea-ice where it was beginning to 

 break away, and waiting until the chunk upon which they 

 were sitting freed itself and floated away northwards. The 

 chicks were thus able to continue their development for some 

 time longer before entering the water. In polar and tem- 

 perate regions floating mats of seaweed carry large numbers of 

 animals about attached to and living in them. Similarly, drift- 



