Page:The Oak.djvu/187

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169
RELATIONSHIPS OF THE OAKS.

are at present, systematists would elevate the latter to the rank of species.

It is interesting to observe that twenty-eight varieties of the common English oak (Q. Rohur) have been described, and that the majority of these can be grouped around the three forms pedunculata, sessiliflora, and pubescens, the latter being a somewhat hairy variety found on the Continent. No doubt we have here, again, a case where the three varieties mentioned would be accorded specific rank if the connecting forms died out, as some of them appear to be doing.

I have already stated that the oaks are a very ancient family, and their great variability is in accordance with this. It probably implies that the genus has had time during its migrations over the Northern Hemisphere to vary immensely, and that some of the varieties have become adapted to given situations, others to others. On the whole, the oak family must be regarded as a northern type which has sent extensions southward.

Now let us glance at their geological history. Something like 200 forms of fossil oaks have been described from remains, chiefly of leaves and wood, found in various parts of the world. Some of the European fossil forms remind us of species now found only in hot countries near the tropics, others are peculiar, and some are very doubtful.

The earliest remains of oaks come from the Cretaceous strata, being coeval with the first undoubted dicotyledons that have been found. Many have been found