SHUT off from eastern North American by the high Sierra wall, that formidable harrier to the eastern and western migration of plant, as well as animal life, and possessing a climate unlike that of any other part of the continent, California has developed a flora that is unique. Indeed, isolation has been so complete that the California flora, with its host of peculiar or endemic species and even genera, displays many qualities characteristic of an insular flora, such as one might expect to find on a remote oceanic island. To the traveler familiar with the flora of the Mississippi Valley or of the Atlantic States, California plants seem as foreign as those of southern Europe. Species of such well known genera as Quercus, Primus and Rhamnus (the oak, the cherry and the buckthorn) are so unlike their eastern relatives in foliage and general aspect that their true relationship is revealed only on close scrutiny.
But if the Sierra wall with its snow-clad summits has been an effective barrier to the eastern and western migration of plants, it has been likewise effective as a pathway for the southern migration of northern plants. And the warm valleys and foothills that lie at its base have been similar pathways for the northern migration of southern types. We find, therefore, the California flora composed of three distinct elements, the Californian, the Boreal, and the Mexican.
The Californian element, as recently discovered fossils prove, was established before the Glacial Period, and through its preservation from the destructive ice sheet, California has been able to hand down such a priceless heritage as the sequoias, an all hut extinct race that at one time flourished over North America, Europe and Asia, extending as far north as Greenland and Spitzbergen. With the sequoias have come clown many other conifers, making the California coniferous forests the richest in the world.
The Boreal or northern element, pushed southward by the ice sheet of the Glacial Period, formed a belt on the California mountains below 5,000 to 8,000 feet, the perpetual snow line of the ice age. At the end of the period, the ice retreated upward and northward, followed by the boreal plants, with the result that we now have arctic and subarctic species stranded on mountain tops a thousand miles or more south of their general range.The Mexican element has migrated, largely since the Glacial Period,