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Quercus Robur offers a familiar illustration of the manner in which one form may in the course of time become separated into two or more distinct ones. To Linnaeus this common oak of Europe was all of one species. But of late years the greater number of European botanists have regarded it as including three species, Q. pedunculata, Q. sessiliflora, and Q. pubescens.
Just as we come to know them better, intermediate forms flow in, and doubts as to specific limits augment." He also adds that it is the best known species which present the greatest number of spontaneous varieties and sub-varieties. Thus Quercus robur has twenty-eight varieties, all of which, excepting six, are clustered round three sub-species, namely Q. pedunculata, sessiliflora and pubescens.
Of the twenty-eight spontaneous varieties of Q. Robur, which De Candolle recognizes, all but six, he remarks, fall naturally under the three sub-species, pedunculata, sessiliflora, and pubescens, and are therefore forms grouped around these as centres; and, moreover, the few connecting forms are by no means the most common.
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