RELATIONSHIPS OF THE OAKS 169 



were to die out, leaving certain varieties more isolated 

 than they are at present, systematists would elevate the 

 latter to the rank of species. 



It is interesting to observe that twenty-eight varie- 

 ties of the common English oak (Q. Robur) 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 accord- 

 ance with this. It probably implies that the genus has 

 had time during its migrations over the Northern Hemi- 

 sphere to vary immensely, and that some of the varie- 

 ties 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 south- 

 ward. 



Now let us glance at their geological history. Some- 

 thing 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 Greta- 



