THE EVOLUTION OF PLANTS
orchids, etc.). This, in bare outline, is the monophyletic hypothesis (Fig. 11).
Other students, on what they believe to be equally good evidence, postulate two primitive main branches, appearing as distinct at the dawn of the fossil record—the club-moss stock, or Lycopsida, and the fern stock, or Pteropsida, from which have descended the modern conifers and flowering plants and their ancestors. This is one of the polyphyletic hypotheses (Fig. 13).
The solution of the question of the “family-tree” of plant life may be roughly likened to the task of putting together a picture-puzzle, in which many of the pieces are not understood and some are perhaps temporarily or permanently lost. If we had a museum collection of specimens of all the kinds of plants that have ever lived, botanists believe that such specimens could be so arranged as to represent their genetic relations and to give us a true picture of the evolutionary development of the present plant world. But probably no such collection can ever be made. We are continually finding, with more or less certainty, where this or that piece belongs in the picture, and lost pieces are continually being discovered as fossils in the rocks or as facts disclosed in the laboratory and field.
Again—to use once more the illustration afforded by the picture puzzle—it is the difficulty of the problem that fascinates the scientist, and it is the modicum of his success that lures him on to further research; he finds his reward in the quest and in the satisfaction of making some contribution, however slight, to the ultimate, but probably unattainable success.
It is one thing, however, to accept evolution as a fact and quite another thing to explain the method of evolution—how this gradual change or series of changes has been brought
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