166 OUR BRITISH PLANTS. 



proportion is confined to the Breadalbane range, and does not 

 occur further south. Upwards of a score of plants found on the 

 Scottish Alps do not reach the English mountains ; while several 

 species are to be met with on Skiddaw and other hills in the north 

 of England, which do not reach the Snowdonian range." We are 

 thus able definitely to ascertain the region from whence this type 

 of vegetation is derived ; but how did the migration take place ? 

 Again geologic discoveries furnish us with an answer. During 

 the great Glacial epoch, which succeeded the deposition of the 

 Tertiaries, the lowlands of Great Britain were submerged beneath 

 a sea which spread over a great part of Europe. In this sea the 

 mountain ridges of the north formed a group of islands, " whose 

 bases and sides were washed by the cold waves and abraded by 

 the passing ice-floes, and whose summits were covered in many 

 places with glaciers. It was at this period that our now elevated 

 regions received the flora and fauna of the present day. Owing 

 to their favourable position in the midst of an ice-covered sea, the 

 means of transport existed in abundance, and the Arctic flora thus 

 brought down has ever since been able to maintain its footing on 

 the high ground which it inhabits." Nor is it Britain alone which 

 owes its Alpine flora to Scandinavia. It is probable that during 

 the same glacial era the allied forms which are found in European, 

 Equatorial, and Antarctic regions travelled from the same far 

 north, and have become modified during the long intervening ages 

 by the altered conditions of existence. 



EXPLANATION OF PLATE. 



One example of each of the four groups of plants is given on 

 Plate XL 



Photomicrographs of Diatoms. — MM. A. Traun and Witt, 

 in their work on the fossil diatoms of Hayti, describe their peculiar 

 method of photographing these objects. They first photograph 

 the diatoms with a magnification of not more than loo diameters, 

 and afterward enlarge the negatives so as to obtain a photograph 

 magnified 500 diameters, proper for photo-printing. Fine details 

 are said to be brought out, which are invisible to the naked eye 

 in the smaller photograph. — The Microscope. 



