Page:Popular Science Monthly Volume 17.djvu/384
370 THE POPULAR SCIENCE MONTHLY.
gested in the article quoted, the mass had come from some African instead of an American river, it still shows one means by which seeds could be dispersed. Wallace gives further confirmation of this fact. He says : " Rafts of islands are sometimes seen drifting a hundred miles from the mouth of the Ganges, with living trees growing on them, and the Amazon, Orinoco, Mississippi, Congo, and most great rivers produce similar rafts. Spix and Martins declare that they saw at different times, on the Amazon, monkeys, tiger-cats, and squirrels being thus carried down the stream. . . . Admiral Smyth informed Sir Charles Lyell that among the Philippine Islands after a hurricane he met with floating masses of wood with trees growing upon them, so that they were at first mistaken for islands, till it was found that they were rapidly drifting along. . . . The fact of green trees so often having been seen erect on these rafts is most important ; for they would act as a sail by which the raft might be impelled in one direc- tion for several days in succession, and thus at last reach a shore to which a current alone could never have carried it." * Now, if such rafts as these were capable of conveying large animals, it would be extremely probable that they would also have on them seeds of many kinds of plants ; and, as we shall see hereafter, as the animals them- selves often convey unintentionally seeds sticking to their coats, they too would be vehicles for their transportation.
Besides the rafts floated down the rivers, it is very probable that those which overflow their banks periodically, as the Nile, the Ganges, Amazon, Orinoco, and Mississippi, or occasionally as many other riv- ers do, would transport seeds from plants growing at their sources to hundreds of miles below. Darwin gives the details of an experi- ment he tried, which illustrates in a remarkable manner the extent to which the mud of rivers and ponds is charged with seeds waiting for a chance to develop. He says: " I took, in February, three tablespoon- fuls of mud from three different points beneath water on the edge of a little pond ; this mud when dried weighed only six and three fourths ounces. I kept it covered up in my study for six months, pulling up and counting each plant as it grew ; the plants were of many kinds, and were altogether five hundred and thirty-seven in number ; and yet the viscid mud was all contained in a breakfast-cup ! " f There can be no doubt whatever that, after inundations of the land by the rivers, plants spring up in localities where they were unknown before, and the inference is just that the seeds were conveyed by the water.
Birds, too, furnish another and important means of transport. Many fruits having a seed incased in a hard shell are surrounded by a juicy pulp : such are the cherry, plum, mistletoe-berry, hawthorn, etc. All these are eaten by birds which, assimilating the pulp, cast the stones in their excrement. The parasitic mistletoe has no way of
- Wallace " Geographical Distribution of Animals," vol. i., pp. 14, 15.
f " Origin of Species," pp. 345, 346.