When we get into the polar regions, all the plants take on a stunted and dwarfed appearance and, in some cases, retire almost entirely under ground. The number of genera and species is much reduced. The Oak, Walnut, Chestnut and Elm are replaced by the hardy conifers. At the point where vegetation becomes almost extinct are dwarf Birches, Willows and polar Blackberries (_Rubus Arcticus_). The simple Mosses and Lichens mark the last lingering evidences of life. A curious feature of plant life in the polar regions is the rapid growth which it often exhibits. The summer of the Far North is short but it is one day of intense and blinding light. The sun shines continually throughout each twenty-four hours. By virtue of its stimulating power, plants are able to perform in a few weeks processes of development which take months under ordinary conditions. It is illuminating to take a single country in a more favoured climate and, as far as possible, trace its plant history. The British Isles, because of their limited area, are a convenient field of study. An investigation of their settlement by plants gives us many hints about prehistoric climatic and geographical changes. Geologists generally believe that the British Isles were once joined to the mainland of Europe. It was at this time that they were settled by vegetation. Some of this plant life came from Spain and some from southwest France; there was also a Germanic group. The floating ice of the glacial period brought over hardy visitors from the Scandinavian peninsula. A few plant immigrants arrived from North America and landed on the west coast of Ireland. St. Helena is an isolated volcanic mass built up seventeen thousand feet from the bed of the ocean. It therefore has its own peculiar vegetation, a portion of which is believed to have been evolved on the spot from the one-celled state. According to Sir Joseph Hooker, forty out of fifty flowering plants and ten out of twenty-six Ferns “with scarcely an exception cannot be regarded as very close specific allies of any other plants at all.” Sixteen of the Ferns are common to Africa, India or America and were probably carried there by the wind. Ocean currents also brought other species from Africa. In 1883, a most interesting thing occurred on the Asiatic island of Krakatoa. A violent volcanic eruption wiped every vestige of life off its surface. When the flow of lava ceased and the earth cooled once more, Krakatoa was to all intents and purposes a volcanic island newly risen from the sea. It presented the exact analogy of a recently created bit of land waiting to be settled by the plants. In 1883, it was as barren as the face of the moon. In 1888, a Mr. Hemsley described its appearance as follows:--