260 EVOLUTION AND GENETICS 



vegetation have been observed far from shore. Beebe records in 

 The Arcturus Adventure a population of fifty-four species on a 

 single log, and in his interesting chapter An Island of Water, in 

 the same work, lists four species of plants, five of birds, eight of 

 shore fishes, four of shore crabs, and eight of insects observed 

 within a space of ten days which might have become established 

 had his "island" been a new-born bit of land. If this could 

 occur in ten days, what might not pass by in a year? With such 

 evidence before us we can hardly imagine land remaining unin- 

 habited. 



Elsewhere the transportation of polar bears on ice floes has 

 been recorded, and the observation of animals of considerable 

 size on drifting masses of vegetation, which are apparently of 

 common occurrence in tropical waters. 



Wind is an important factor in the dispersal of animals of 

 small size, and an aid to the dispersal of volant forms in particular, 

 whatever their size. Insects are especially likely to be transported 

 in this way, for their bodies are so small and light that they are 

 unlikely to fly strongly enough to resist high winds. It is not 

 uncommon to find battered specimens of butterflies a hundred or 

 more miles to the north of their normal range, often after a period 

 of southerly winds. These individuals, because of highly special- 

 ized food habits, do not usually establish the species in the region 

 to which they are carried. In some cases, however, a species 

 appears occasionally on cultivated plants which is not normally 

 seen. Its intermittent occurrence may well be due to gravid 

 females carried by flight and wind. Such forces have no doubt 

 been responsible for the peopling of oceanic islands with organisms 

 similar to those of the nearest mainland. 



Plants are in many cases specially adapted for wind dispersal 

 through the production of buoyant seeds. Wing-like expansions 

 and tufted appendages, such as those of maple and milkweed seeds, 

 are familiar examples. 



Flowing waters, either ocean currents or streams, act in a similar 

 way, but they can act only on aquatic organisms or in connection 

 with some buoyant object. The seeds of plants may be trans- 

 mitted in this way usually without special adaptation, since many 

 of them are incidentally light enough to float. Some are aided, 

 however, by buoyant accessory structures, such as the fibrous 

 husk of the cocoanut and the receptacle of water liUes. 



