70): can we look to these swift messengers for assistance? The rapid digestion of birds renders it futile to expect that even those which do not crush the seeds which they eat could bring over from the Pyrenees seeds which they have swallowed; so we are forced back on the uncertain method of ectozoic dispersal: that is, on the assumption that seeds of these plants have been imported by becoming entangled in the feathers of birds, or by adhering--possibly with the aid of mud--to their feet. That seeds are transported by these means has been shown by the observations of Darwin and other observers; but that the seeds of a number of different plants, growing in different situations, should be brought thus from the Pyrenees and Mediterranean to our western coasts is a highly speculative suggestion. If we discard it, there is left the hypothesis that the plants migrated long ago overland, at a time when the western coastline of Europe was continuous and lay farther seaward. Such conditions have not occurred since the Ice Age; so we have to assume that the plants, arriving perhaps in Pliocene times by slow terrestrial dispersal, and subsequently cut off by invasions of the sea upon their line of advance, survived the cold and ice of the Glacial Period within the limits of our islands. That appears, on consideration of the geological evidence of widespread glaciation, sufficiently improbable; but we must remember that the evidence supplied by the plants is buttressed formidably by that of the corresponding animals, some of which, such as the Kerry Slug, are far less fitted for transmarine dispersal than are the seeds of plants. Also, we are faced with the problem of the American plants, and such organisms as the American Sponge, _Heteromeyenia_: a direct crossing of the ocean appears for them wholly impossible. Yet if they crossed over long-gone land surfaces, their arrival on this side of the Atlantic must be very ancient, and they must certainly have weathered successfully the Great Ice Age. The problem, it is clear, is an exceedingly difficult one, upon which it would be rash to pronounce any hasty opinion. Students of the subject have come to widely difficult conclusions: some holding with Edward Forbes that these Lusitanian and American organisms represent the very oldest element in our fauna and flora, having migrated over bygone land surfaces in distant times and successfully survived the terrors of the Glacial Period; others claiming a much less remote period for their immigration. Indeed, one eminent recent writer on the subject, the late Clement Reid, considered that the Lusitanian plants are among the most recent arrivals in the country, their introduction being due mainly to birds driven by exceptional gales.