While the cause of mutants has not yet been revealed, we have already seen that the two remaining kinds of variability are due to changes in the environment, or to crossing. Charles Darwin when he published his “Origin of Species,” than which no other book has so completely revolutionized modern thought, did not state the cause of those variations of which he was our greatest observer. He did state the now universally accepted law of the “Survival of the Fittest” which explains how, once these variations make their appearance, the inexorable conflict of nature would automatically weed out the unfit. We have seen all through the course of this history of the plant kingdom how whole types of vegetation have been overthrown to give way to other types better fitted to survive. That process is going on with just as inexorable results to-day as it has down through the ages. While Darwin never claimed that such a purely selective process could initiate new species, many of his partisans who waged battle for him during the first years of the tremendous opposition his views encountered, did so claim, and probably wrongly. The actual cause of the origin of new species, except those demonstrated to result from new combinations of already known characters, through crossing, cannot be explained through the natural selection of plants or animals which exhibit favorable variations. We see their effects; it is obvious enough, that those of value tend for the survival of plants having such variations, and it was natural enough that older students of the problem should mistake these effects for the cause of them. The process of selective elimination constantly going on does tend to fix certain favorable variations and untold millions of plants have had their day in the past due to their possession of such, and the killing off of their less fortunately provided associates. We speak of this great march from the simplest organism up to our most complex plants and animals as their evolution, but we must never forget that it has gone step by step, by one or the other methods by which we have seen that plants vary, or perchance by some undetected method, and that while the results of it are for all to see, the causes of that infinitely slow and quite often wayward variation are not understood. Upon such a conception our modern plant life is seen to be a development of plants that have gone before, that all existing life is derived from preexisting, and not from providential interposition or special creations. All through the long marches of plant evolution there appears to be a definite and final goal toward which it tends, but we do not know the direction, least of all the object, of that goal. In fact, there may be many goals, just as there are the diversity of ambitions among human beings. In tracing the present ascendancy of our flowering plants from their links with the past perhaps we shall find no better statement of their present condition or destiny than to repeat Cardinal Newman’s reply: “To live is to change, and to be perfect is to change often.”
CHAPTER VIII
DISTRIBUTION OF PLANTS
WE have seen in the previous chapters how many and how varied are the activities of the plant world and in this final one we shall get a glimpse of what these activities have produced. All the delicate mechanism of food getting and the manufacture of starch; the fertilization of flowers by insects, the wind or water; the response to changing light and to climate—these and scores of other activities of plants have resulted in the present vegetation of the world being what it is. Here we see the final reflection or register of not one but all the kaleidoscopic evidences of plant response and activities and history working in harmony, or, as we shall see presently, sometimes in violent conflict, and leaving as the result the wonderfully varied vegetation that now covers the earth. If we could read aright the story of which the vegetation of any particular country is the silent narrator, it would tell us not only what happened in the past but what is likely to happen in the future.
Plants, by what amounts to a kind of fatality, are rooted to the spot where they grow so that, unlike animals, their rapid distribution appears to be almost impossible, and yet the tremendous distances that some species have traveled seem like a pretty successful protest against the fact of the anchorage of individuals to the point of their origin. It is more than a successful protest, for it amounts in many species to an active campaign for dominance, to the exclusion or extinction of less aggressive neighbors, so that in any field or meadow or forest there are silent struggles constantly going on. Some of these are so inexorable in their results that they change not only the frequency of occurrence of the individuals involved, but sometimes the whole type of vegetation.
The competition to occupy just as much of the favorable plant sites as possible has been much aided by many species possessing means for the dispersal of their seeds or fruits that are ingenious in the extreme. Some of these are written plainly enough in the structure of the seed and its wonderful adaptability for the peculiar conditions to which it will be subjected. Before considering some of these structures we may profitably see how some plants look after the dispersal of their seeds within their own limited sphere of action.
In hundreds of plants the ripened pods, instead of being erect as their flowers have been, are pointed downward about the time the seeds are ready to be released, and their harvest is sown, sometimes by deliberate movements, in the immediate vicinity. No great areas are captured by such plants, except by the slow process of successive generations extending their range a few inches or at most a few feet a year. The great bulk of all seeds never do grow into new plants, but in those that only shed their seeds close to the parent plant the opportunity to reach new sites is by that much restricted. The chance of the species getting very far afield except by slow invasion of the neighboring region is limited. A few of such plants show remarkable ingenuity in reaching the utmost distance possible, perhaps the most effective cases being those that shoot their seeds by explosive bursting of their pods. Nearly all the violets do this, often shooting seeds several feet from the parent plant. Many plants of the pea family have pods that are twisted, which upon splitting release the previously pinched seeds so suddenly that they are shot considerable distances. In the common witch-hazel ([Figure 111]), the seed is shot through the air often as much as thirty feet.
