574 TRANSACTIONS OF SECTION K. 



More than sixty years ago a great impetus was given to this study by the 

 discovery that Europe had passed through a most remarkable series of climatic 

 changes, and that, too, during the lifetime of the existing species of animals 

 and plants. There had not been a mere cooling of the climate; the temperature 

 in these latitudes had sunk far below its present level, and then had again 

 risen. 



Edward Forbes, in 1846, seized this clue, and explained through it, as relics of 

 the Glacial Period, the arctic plants stranded on our mountain -tops ; they were 

 plants left' behind when the climate became too warm for them any longer to 

 survive on the plains. The subsequent discovery of fossil remains of these 

 plants scattered over the plains, and often associated with relics of arctic 

 animals now extinct in Britain, seemed a brilliant proof of Forbes' view, which 

 has been generally adopted. 



In some curious way, however, botanists and zoologists both seem to have 

 overlooked the difficulty that, granting Forbes' hypothesis to be sufficient to 

 account for our alpine flora, it rendered more difficult instead of easier the 

 explanation of our southern flora, which occurs in a similar way stranded in 

 some of the warmest low-lying parts of Britain. 



We meet to-day to discuss this question, in the hope that botanists, zoologists, 

 and geologists may realise each other's difficulties, and may be able in combina- 

 tion to give a clear teaching on this important problem of geographical distri- 

 bution. 



The discussion I have been asked to open is limited to the relation of the 

 present Plant Population of the British Isles to the Glacial Period. Our prob- 

 lem is a special one ; it is not the same as that which confronts the botanist 

 on the Continent of Europe or America; and it is not the problem of the origin 

 of the flora of an oceanic island. Also, the wider question of the origin of the 

 species composing the British flora is outside the discussion, for it would lead 

 us into too many untrodden bypaths, and could not satisfactorily be gone into 

 in the present imperfect state of our knowledge. 



Perhaps it will be well to explain at once why the inquiry is thus limited to 

 comparatively recent periods, and how it is that we need not explore the unknown 

 earlier periods and deal with larger questions. 



Our first inquiry in this case must be : Has there been any continuous occu- 

 pation of Britain by a temperate flora and fauna from pre-Glacial times to the 

 present day ? Or, to put it in other words : Are any of our plants survivors that 

 managed to live through the cold of the Glacial Period in some warm nook in 

 Britain ? They evidently found a refuge somewhere, for we know that the same 

 temperate species that live in Britain now were here in pre-Glacial times. But 

 was this refuge in Britain ? 



Here geology comes to our aid, and I think that all geologists who have made 

 a special study of the climatic conditions will agree with me. Any survival of 

 our flowering plants, except in the case of a few arctic and alpine species, was 

 quite impossible. 



It may come as a shock to some of my colleagues when I say that for this 

 particular discussion we have a perfectly definite starting-point. We have 

 merely to account for the incoming of our existing flora, after an earlier assem- 

 blage had been swept away almost as completely and effectually as the celebrated 

 volcanic eruption wiped out the plants of Krakatoa. 



In order to make clear the existence of this limitation, and for the con- 

 venience of the discussion, I have prepared certain maps, which are now shown. 

 I propose now to say a few words as to the bygone climatic and orographic 

 changes indicated on those maps, and on their bearing on the existing flora of 

 Britain. I must say at once, however, that you must not take these maps as 

 absolutely exact statements as to the climatic and geographic conditions at the 

 different stages involved in our inquiry. But they give the result of many years' 

 work at this subject, and, I think, may be accepted as embodying the main 

 factors which dominate the question we have to discuss. 



We know that during the greatest intensity of the cold all Scotland, Ireland, 

 and the greater part of England were buried under ice and snow— except, pos- 

 sibly, for some high peaks on which a few arctic species survived. Ice filled 

 the North Sea and covered the lowlands of England down to the mouth of the 

 Thames. Without crossing the Thames it almost reached London. Its southern 



