[Illustration: Fig. 66. Intolerant Tulip. Notice the long, straight boles. _U. S. Forest Service._] The distribution of a species may also be determined by geographical barriers, like mountain ranges and oceans. This is why the western forests differ radically from the eastern forests and why the forest of Australasia is sharply distinct from any other forest in the world. Any one or several of these factors, soil, moisture, heat, and light, may be the determining factor in the make-up of a forest, or it may be that a particular tree may survive, because of a faster rate of growth, thus enabling it to overtop its fellows and cut off their light. The struggle for survival is constant, and that tree survives which can take the best advantage of the existent conditions. Besides these topographical and climatic factors which help determine the distribution of trees, a very important factor is the historical one. For example, the only reason by which the location of the few isolated groves of big trees in California can be accounted for is the rise and fall of glacial sheets, which left them, as it were, islands stranded in a sea of ice. As the glaciers retreated, the region gradually became re-forested, those trees coming up first which were best able to take advantage of the conditions, whether due to the character of their seeds, their tolerance, their endurance of moisture or whatever. This process is still going on and hardwoods are probably gaining ground. Besides these external factors which determine the composition and organic life of the forest, the trees themselves furnish an important factor in their methods of reproduction. These, in general, are two, (1) by sprouts, and (2) by seeds. (1) Most conifers have no power of sprouting. The chief exceptions are pitch pine and, to a remarkable degree, the redwood, Fig. 67. This power, however, is common in broad-leaved trees, as may be seen after a fire has swept thru second growth, hardwood timber. Altho all the young trees are killed down to the ground, the young sprouts spring up from the still living roots. This may happen repeatedly. Coppice woods, as of chestnut and oak, which sprout with great freedom, are the result of this ability. The wood is poor so that it is chiefly used for fuel. [Illustration: Fig. 67. Sprouting Redwood Stumps. Glen Blair, Calif. _U. S. Forest Service._] (2) Most trees, however, are reproduced by seeds. Trees yield these in great abundance, to provide for waste,--nature's method. Many seeds never ripen, many perish, many are eaten by animals, many fall on barren ground or rocks, and many sprout, only to die. The weight of seeds has much to do with their distribution. Heavy seeds like acorns, chestnuts, hickory and other nuts, grow where they fall, unless carried down hill by gravity or by water, or scattered by birds and squirrels.