FLORA OF THE WEST YORKSHIRE HILLS. 265 



fixed temperature has been reached, or a certain fixed height 

 attained, plants have bounds set to their upward or downward dis- 

 tribution, over which they cannot pass. On the contrary, where 

 the conditions — such as presence or absence of moisture, soil, 

 aspect, shadow, or sunshine— are more favourable to them, many 

 plants are able to grow higher up or lower down one hillside than 

 on another. Limosella aquatica, Hippuris vulgaris, Potamogetoji 

 densus, usually lowland forms, may be found thriving at a height of 

 from thirteen hundred to fifteen hundred feet on Malham Moor, 

 and Saj?iolus Valerandi, another lowland plant, at a height of one 

 thousand feet, near Sheffield. The following mountain forms 

 descend : — Draba mcafia to one hundred and fifty feet, Gnapha- 

 Hum dioicum to one hundred feet, Polypodium calcareum to two 

 hundred and fifty feet, Actcea spicata, Alyrrhis odorata, Sagina 

 nodosa, Ei7ipeirum 7iigrum, and Lastrcsa oreopteris to below one 

 hundred feet. 



In considering the flora of the Yorkshire hills, a distinction 

 can therefore be made between plants preferring, and being more 

 fitted for, the plains, and yet capable of growing and thriving at a 

 greater or less elevation on hills, and plants flourishing best on 

 bleak altitudes, yet descending to lower levels, the one set en- 

 croaching on the domains of the other, so that no distinct line of 

 demarcation can be drawn between them. Further, as has 

 already been stated, the limits of extension of any species, either 

 in an upward or downward direction, varies in different localities, 

 the limits being governed by influences to which plants, as living 

 things endowed with a certain amount of ability to adapt them- 

 selves to circumstances, have responded ; for they, like animals, 

 have long been subjected to the influences of the external condi- 

 tions of their environment, and these were not, and are not, always 

 of a material kind, like local peculiarities of soil, situation, tem- 

 perature, and degree of moisture. Those organisms, which have 

 failed to respond to the requirements of their external surroundings, 

 have been weeded out by death, leaving only those plants, which, 

 through the vigour of their inherited constitution, produce hardy 

 seeds, which, in turn, by virtue of their innate vitality, take pos- 

 session of the waste places of the earth to the exclusion of their 

 frailer brethren. These directing forces are still operative, and 



