"The pine trees," says Muir, "march up the sun-warmed moraines in long hopeful files, taking the ground and establishing themselves as soon as it is ready for them."
[Illustration: _From the painting by Rousseau in the Metropolitan Museum of Art._
Last of all come tramping along the sturdy old oaks.]
Last of all come tramping along the sturdy old oaks and the nut-bearing trees. Their seeds are so heavy they get little help from the winds, and then only in the most violent storms. They must advance very slowly indeed, with occasional help from absent-minded squirrels who carry away and bury nuts and acorns and then forget where they put them.
[Illustration: HOW SQUIRRELS HELP OAKS TO MARCH
Sometimes they bury acorns and forget just where. When frightened they often drop them and run away.]
The beginnings of a forest are stunted because the soil is thin. Moreover, the company in which the trees find themselves is very miscellaneous, like the population of all pioneer communities--weeds, grasses, briers, shrubs. High up on a mountainside you can find all these types of vegetation. Pines growing clear to the snow line; farther down the mountain, in crannies, sumach and elder bushes with field daisies and goldenrod scattered among them; while on the barren rocks are the lichens and the mosses.
Not only do the citizens of the plant world follow a certain fixed order in coming into new regions, but also in giving place to one another. All plants of a higher order can live only on the remains of those of a lower, and it is most interesting to note the process by which each lower form comes, does its work, passes on, and is replaced by a superior type. The shrubs, which can only grow after the weeds and grasses have made enough soil for them, at length shade out these smaller pioneers. Haven't you often noticed, when picnicing in deep woods, that the grasses and flowers are to be found only in the sunny spaces, where there are no trees?
But these thickets themselves, after a while, disappear, and pines take their places. I am speaking now of the growth of forests, where the soil-making has so far advanced that forests are possible. The thickets, with their good soil and the shade which keeps it damp, are just the places for the pine seeds brought in by the wind to get a foothold and sprout up. When they grow into big trees they gather with their high branches so much of the sunshine for themselves that little of it gets through to the shrubs below, so these shrubs disappear, surviving only in the sunny open spaces or along the borders of the wood.