PAST GEOGRAPHICAL MUTATIONS 25 



was not only cold but extremcl}- humid ; vast forest 

 areas of oak, alder, and other deciduous trees were 

 destroyed, and the ground converted into marshes and 

 bogs, not only in the British Isles, but in Northern Ger- 

 many and throughout North-western Europe. Event- 

 ually this cold, humid period passed away, and a second 

 era of more favourable conditions supervened, during 

 which a vigorous growth of forests (chiefly pines and 

 birches) flourished. This era was marked by a gradual 

 retreat of the sea. The subsequent destruction of many 

 of these later forests and their burial in sphagnum peat 

 mark a relapse to another period of cold, humid con- 

 ditions, accompanied by a rise of sea-level and sub- 

 mergence of much forest-clad area in many maritime 

 districts. To this latter period most of the submarine 

 forests of the British coast-line are said to belong. With 

 the passing away of this cold, wet period we approach 

 the earliest ages of written history, the invasion of 

 Britain by the Romans, the climatal conditions and 

 arboreal surroundings which are characteristic of pre- 

 sent time. The effects of these gradual yet enormous 

 changes of climate upon the migration and distribution 

 of birds in our area will never be known. That these 

 changes must have influenced the fauna as well as the 

 flora is unquestionable, but unfortunately these are 

 details of our inquiry which will probably always remain 

 beyond the limits of human knowledge or conjecture. 



