Yet there are limits to its cosmopolitan endurance. Not long ago I spent a profitable day in the Arnold Arboretum at Boston, Massachusetts, under the guidance of its presiding genius, Professor C. S. Sargent. After wandering for hours amid the luxuriant vegetation of that magnificent park, we stopped beside a mangy, stunted conifer, and he asked me whether I recognised it. I did not; but guessed at hazard that it was the Japanese _Pinus parviflora_. I was surprised to be told that this was the best that could be done in that country with our own Scots pine. From causes difficult to define, probably similar to those which prohibit the growth of our common ivy in the Eastern United States, this tree resists all attempts to make it at home in that atmosphere. [Illustration: SCOTS PINE WOOD] It may seem strange that this tree should be known as the Scots pine, having regard to its enormous geographical range and to the insignificant area which it occupies in Scotland as compared with the vast forests in Russia, Scandinavia, and other countries. Its scientific title, _Pinus sylvestris_--the forest pine--would appear more appropriate. But it has received its English name because, although at one time it was spread as a native over all parts of the British Isles, it is now only to be found in a truly wild state in the fragments of old forest remaining in Strathspey, Deeside, and here and there in the counties of Inverness and Perth. From England probably it had entirely disappeared when, in the seventeenth century, certain landowners succeeded in reintroducing it; and now it has attained splendid proportions in Surrey and other southern counties, and spreads freely by its winged seeds wherever these fall on unoccupied lands. Were it not for deer, sheep, and rabbits, most of our _dry_ moors and heathland would be covered with pine forest up to the thousand feet level. Howbeit, most of the moorland in the United Kingdom is the reverse of dry. Except in Eastern Scotland and the Surrey uplands, it is usually clad with a dense coat of wet peat, reeking with humic acid and inimical to tree growth of any kind. One of the darkest enigmas of natural science is presented in the remains of pine forest buried under such a dismal treeless expanse as the Moor of Rannoch, and on Highland hills up to and beyond 2000 feet altitude, far higher than any tree can exist now. The explanation seems most likely to be arrived at in the direction indicated by certain symptoms of the alternation of periods of greater and less rainfall--periods comprising thousands, perhaps tens of thousands of years. Trees, it has been suggested, might grow and reproduce themselves at high altitudes during the drier cycles; but when the rainfall and atmospheric humidity increased beyond a certain degree, the soil would become covered with moss, seedlings would be smothered or never start, and humic acid would render the ground unfit for any growth except heather and moorland herbs.