CHAPTER V ORGANIC EVOLUTION 
THE morphology of plants, as presented in the preceding chapters, is in 
reality a somewhat detailed illustration of the evolution of the plant 
kingdom. The theory of descent is the working theory of modern biol- 
ogy, and no student of morphology should omit some consideration of it. 
The subject has developed so extensively that it can be presented here 
only in very brief outline, an outline that may serve as an extended defi- 
nition, and also as an introduction to a real study of organic evolution. 
Many names connected with the doctrine of evolution and many impor- 
tant views in reference to it must be passed over, and only the most con- 
spicuous features presented. 
. Definition. The doctrine of organic evolution claims that the exist- 
ing plants and animals are the modified descendants of earlier forms; 
that in some way new forms have arisen from the old ones, and have 
given rise in turn to still other forms. According to this view, the whole 
plant kingdom, for example, may be likened to a profusely branching 
tree, the tips of whose myriad branchlets represent our present flora. 
The morphologist attempts to trace these branchlets from their tips, 
which he sees, to their connections, which he can only infer. His proofs 
are obtained from the structure and development and behavior of living 
plants, and also from the form and structure of ancient plants, so far as 
they are available in suitable fossil form. His conclusions, it must be 
remembered, are reasonable inferences, and cannot be based upon actual 
demonstrations. It is evident that opinions may differ widely as to the 
actual historical connections of plant groups; but it is practically unani 
mous that there are such connections. 
The idea of organic evolution is not modern, being in the thought of 
man as far back as records of thought have been found; but it is only 
in modern times that it has been based upon direct observation of the 
facts, that is, has become scientific. It is not necessary to recite the 
facts that underlie the widespread belief in organic evolution, for many 
of these facts, so far as they concern plants, have been given in the 
preceding chapters. To believe in organic evolution,, however, is one 
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