One of the South Down sheep is trailing behind it a piece of bramble which has caught in its wool; others, which have been grazing on the broken cliff-edge where Agrimony, Forget-me-not, and Burdock are flourishing, are certain to have spiny or sticky fruits entangled in their wool. Animals therefore carry seeds in their wool or fur. If it should happen to be a fine, sunny afternoon, and if there are any plants of Broom near by, it is quite likely that you may, every now and then, hear a faint, sudden crack. This will be the Broom at work scattering its seeds by itself. The little pod, when it dries, contracts in such a way that it splits with a sudden explosive pop, and the seeds are sent flying to a distance of three or four feet. This curious fact was observed in 1546 by the naturalist Boek. The Whin and many other plants act in the same way, for the dry fruit becomes elastic and coils up spirally, flinging away the seed. But here also, on the southern shore of England, we are at a main station of arrival and departure for migrating birds. A Landrail or other marsh bird might be flushed in France, and might quite easily cross the Channel with French mud sticking to its plumage. In this mud, or in its crop, there may be seeds or fruits which will be left in an English pond. This method is probably a very important one, for these plants growing in duck-haunted places are amongst the most widely distributed of all. Mr. Reid has a very interesting discussion on this point. The crow or rook could quite well cross the British Channel now. In the days when Britain was covered with ice and snow, the gap between the French and the English shore was only half the present width. There was at that time much flat land with oak forest bordering the French coast. Mr. Reid shows that it is probable that rooks regularly carry about acorns in the cup, for he found seedling oaks associated with empty acorn husks, stabbed and torn in a peculiar way. "On October 29th of 1895, in the middle of an extensive field, bordered by an oak copse and scattered trees, I saw a flock of rooks feeding and passing singly backwards and forwards to the oaks. On driving the birds away, and walking to the middle of the field I found hundreds of empty acorn husks and a number of half-eaten, pecked acorns."[113] So that crows may have brought the acorns that colonized Britain with oak forest in the earliest historical period. [113] Reid, _Origin of the British Flora_. Another means of dispersal is not so obvious on the South Downs. In the Arctic region a glacier breaks away at its tongue into icebergs, which float off and are stranded somewhere perhaps hundreds of miles distant. Upon these icebergs are stones and soil and plants which may be carried to a great distance from their original place. In the Glacial period or Great Ice Age, ice may have been an important help in distributing plants, but at present it is difficult to find a good example.