268 FLORA OF THE WEST YORKSHIRE HILLS. 



which determines the limits of their distribution ; nevertheless, 

 they form the characteristic vegetation of high altitudes not only 

 in Yorkshire, but elsewhere in Britain. Some of them are surviv- 

 als of the Glacial Period, when great ice-floods filled the Yorkshire 

 dales to a height of fourteen hundred or fifteen hundred feet, and 

 are, therefore, amongst our oldest floral types ; but the greater 

 number are supposed to have spread to England, across former 

 land connections, from districts not ice-bound at a period of time 

 immediately subsequent to the Ice Age. 



The surface of this country, just emerging from its ice-cap, 

 would present an appearance somewhat similar in character to that 

 of the present-day Tundras of the north of Russia. The incom- 

 ing plants spread over the whole of the country, but as the climate 

 ameliorated and the soil improved, they withdrew from the culti- 

 vated tracts and made their home on the elevated moorlands, 

 where they are still to be found {e.^^., Ejiipetrum nigrum, Jiincus 

 squarrosHs, Galmm saxatile). 



By virtue of their descent, they are endued with a nature 

 capable of enduring very great cold, and flourish best at or near 

 the summits of hifls where alone they can obtain habitats^ approx- 

 imating most nearly in climatic conditions and soil to the frozen 

 regions of the North, where they have their headquarters. This 

 fact, however, does not hinder them from descending to lower 

 levels, where the temperature is higher, so long as they can obtain 

 suitable soil and sufficient moisture. Then, again, they are less 

 numerous, both as species and individuals, than the ascending 

 series. This arises from the restricted choice of fitting situations 

 in which to grow, due to the smaller area of surface which is open 

 to them. They are not subject, therefore, to such fierce compe- 

 tition as the rank-growing plants of the plains, and have not 

 become habituated (if I may use the term) to conditions implying 

 a struggle for existence against their fellows. Though cold suits 

 their constitution, they do not fear warmth so much as they do 

 competition, and meeting and mingling on their downward course 

 with the ascending plants accustomed to the latter condition, 

 cannot maintain their ground, and fail to establish themselves. 



The following are illustrated selections from the Descending 

 Species of plants in Yorkshire : — 



