OUBinr NATUBAL HISTOBT 80C1£TT. 101 



recorded stution, and Cumailte Mountains, county of Limerick, its. most 

 northern), and as far east as about 7° 50' W., the Blackwatcr valley re- 

 presenting the latter; the plant growing in greatest perfection, that 

 18, fruiting most regularly and perfectly at Iveragh, and in the valley 

 of the Blackwatcr; the extreme humidity of tlie Killamey district, 

 whilst encoui-aging the ordinary growth of the plant, interfering with its 

 fruiting. 



Outside this well-marked district are two outlying stations, both, at 

 the time of their discovery, sickly, and now extinct, or at least one of 

 them : one, Wicklow, situated in about 52° 10' N. and 6® 20' W. ; the 

 other, Bellbank, near Bingley, Yorkshire, about 53° 30' N. and 1° 55' W. 



In all these districts Lophod. Fomesecii is found luxuriant, but local 

 in the latter two. 



Suppose these two plants to have made their entry into Ireland 

 somewhere in that district YfYiiohAsplenium lanceolatum, A.acutum, Adi- 

 antum capillus Veneris mark out as the head-quarters of the Lusitanian 

 Flora, and to have spread under favourable circumstances as far north as 

 Yorkshire ; that then, the two countries severed, the physical conditions 

 of district were so altered as to destroy the balance necessary to the 

 former's existence in the interv^cning districts. This favoured spot at 

 Bingley, where Eichardson gathered, one hundred and fifty years ago, 

 the celebrated specimen which has so puzzled botanists, remained as the 

 last haunt of Trichomanes radicam ; whilst its hardier compatriot escaped 

 the general destruction of Lusitanian forms so well as to supply us with 

 abundant stations even much further north than this. Similarly the 

 Wicklow station may be but an offset from that in the county of Water- 

 ford, and may be even anterior to it, the chain of continuity being 

 broken by some causes which caused the disappearance, plant by plant 

 and station by station, of Trichomanea, so that the sickly plant discovered 

 by Dr. Whitley Stokes and Miss Fitton, in 1805, at Powerscourt 

 Waterfall, at last alone remained to point out the old colonization of the 

 district with the species. 



Some explanation of the set of terms already freely used is ne- 

 cessary, viz., " balance necessary to the plant's existence." By this is 

 meant those degrees of intensity of physical agencies, especially light, 

 heat, and moisture, compatible with the existence of any species, or, as 

 we might otherwise express it, the physical range of its existence. The 

 following series of laws express this more generally : — 



1. Every species requires light, heat, moisture, &c., for the due per- 



formance of its functions, and the quantity of these thus neces- 

 sary may vary within certain fixed limits. 



2. That this limited standard may be still further either increased 



or diminished within certain further limits, without destruction 

 of the life of the species, though usually at the expense of or 

 deterioration of some of the functions. 



3. That the (1) standard of growth and (2) range of existence varies 



in species springing from the same centre. 



