CH. Ill] DISPERSAL OF OLIGOCH^ETA. 139 



out at sea and they often harbour even large mammals, 

 which may thus reach a comparatively distant spot ; but 

 unless the water remained absolutely calm during the 

 long period necessary for the drifting by currents so that 

 no splashing occurred the worms would probably be killed. 

 Icebergs on the other hand, which often rise high out of 

 the water, might conceivably be efficient vehicles for the 

 transference of these animals. Prof. Leidy found a small 

 worm frozen in a block of ice which recovered ; the worm 

 was a member of the family Enchytraeidse, but it is possible 

 that a small earthworm with a thick body wall might also 

 survive temporary freezing. The most active agents in 

 the transference of small animals from country to country 

 are however birds ; but in this case they could be of but 

 little use. The only way in which a migration of this 

 kind could be effected would be in the cocoon; earth- 

 worms invariably deposit their eggs in chitinous cocoons 

 from which the young do not emerge until nearly adult. 

 But these are often deposited deepish in the ground, or at 

 the roots of grass, whence they would not be very likely to 

 be detached by, and to stick to, the feet of birds. The 

 case however is different with the aquatic Oligochaata. 

 These worms are in the first place smaller than the 

 majority of earthworms, with smaller cocoons, which could 

 be more readily transported ; in the second place they are 

 deposited often at the margins of pools, where Limicolae, 

 ducks, and other birds are in the habit of dabbling. The 

 possibilities of successful migration by these means is 

 increased by the fact that whereas in earthworms it is the 

 rule for one or at most a limited number of individuals to 



