There can be no doubt that birds eat and subsequently eject vast numbers of seeds still capable of germination; many observations and calculations might be quoted. But when we come to apply the facts to the problem of long-distance dispersal, or the passage across serious barriers, we find that important limiting factors must be taken into account. The digestion of birds is remarkably rapid, food being ejected from a half to three hours after being eaten, so that a bird eating seeds and at once flying off in a straight line at, say, 50 miles per hour could not convey seeds more than 150 miles. Secondly, many observations show that on migration birds generally travel with empty stomachs and clean plumage and feet. It is clear, therefore, that, as in the case of wind dispersal, we must look to exceptional circumstances, not normal conditions, to provide opportunities for long journeys on the part of seeds. But for the transfer of seeds from France to England, for instance, or from England to Ireland, it is clear that birds furnish a far more efficient medium than wind or water. In one important particular, dispersal by animals has a great advantage over dispersal by wind--that it is practically independent of the weight of the seeds. Thus, the heaviest of British seeds, the acorn, is carried about by Rooks, just as the hazelnut is scattered by Squirrels, or a head of Burdock fruits by a passing sheep. Having thus arrived at some idea of the high efficiency for dispersal of many kinds of seeds, it is with some little surprise that we observe--as we may on any country walk--that the plants which arise from these are in general no more abundant or more widely distributed than others which possess seeds devoid of any apparent advantages in this respect--seeds which cannot fly nor float, nor cling to a passing creature, and which are not eaten to any extent by birds so far as observation goes. The truth is, we have to remember, as emphasized in a previous chapter, that the world is already densely populated by plants, all of which survive by reason of their being specially fitted for their several habitats. They have fought in the great struggle for existence, and have established their right to the places which they occupy; they will not readily give way to any newcomer whose seeds happen to be imported into their strongholds. Of course exceptions can be quoted, where plants accidentally or intentionally introduced by man into new areas have not only maintained a foothold, but have spread remarkably. Note the case of the Sweetbrier (_Rosa eglanteria_) in New Zealand, of the Mexican _Bryophyllum calycinum_ in many Tropical countries, of the American Monkey-flower (_Mimulus Langsdorfii_) in our own islands; but these are admittedly exceptional. It is nearer the truth to say that the troubles of an immigrant only begin where dispersal ends; and that the chance of seeds carrying out a successful migration is much greater than the