This seemed to be true, so long as a genus was imperfectly known, and its species were founded upon a few specimens, that is to say, were provisional.

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.