4] DISPERSAL AND MIGRATION II7 



as Bog-bean {Menyanthes trifoliata) and Pondweeds, that have been 

 fed to Perch and Roach, nearly all germinated after being retained in 

 the viscera for one to three days before being passed naturally. These 

 fish are liable to be eaten by such predators as Fishing Eagles, 

 Herons, and Pelicans, which, after an interval of many hours, either 

 reject any contained seeds in pellets or pass them in excreta — often 

 still in a viable condition, as was shown by Darwin. The same 

 doubtless happens to many Algae, aquatic Fungi, etc. By the time 

 such plant material is ejected, the carrying bird may have flown 

 many miles. Some of the larger aquatic Crustacea and MoUusca as 

 well as, of course, Reptilia, obviously play a part in the dispersal of 

 Algae which grow epizoically upon them or their shells ; while on 

 land. Snails and Slugs disperse seeds and spores that adhere to their 

 bodies or have been swallowed. Indeed, it is said that the spores 

 of some Fungi will only germinate after passing through a Slug ; and 

 when the latter is eaten by a Toad, Bird, or other predator, the 

 possibility occurs of far more extensive dispersal. 



Insects are probably the most important of the groups of lower 

 animals in the matter of plant dispersal, especially of very small 

 bodies such as fungal spores. Transport is commonly by swallowing 

 and ' passing ' in the excreta, by carrying to their nests for food, and 

 by adhesion. Locusts are said to afford examples of the first method, 

 sometimes over considerable distances, and ants frequently transport 

 seeds with edible appendages, while flies and many other insects 

 often carry spores of cryptogams adhering to their bodies — especially 

 when the latter are densely hairy. Further instances are the well- 

 known transmission of Fungi- and Bacteria-engendered diseases by 

 insects, as well as important viruses (such as those of Potatoes) 

 having aphid vectors. 



It seems desirable here to treat briefly the subject of pollen. 

 As we saw in Chapter II, pollen is composed of vast numbers of 

 microscopic ' grains '. These, though capable of producing on 

 germination only a tinv particle of plant, and hence scarcely to be 

 considered as true disseminules, nevertheless carry the potential 

 male gametes and, in them, the genes introducing hereditary 

 characters. As it is now known that transport of pollen can in some 

 circumstances take place naturally over many hundreds of miles, 

 and that given suitable conditions some detached pollens can live 

 for manv months, it seems conceivable that by this means heritable 

 characters mav be transported vast distances. To be sure, the 

 grain has to find its way to a receptive female stigma to have any 



