100 DUBLIN NATURAL HISTORY SOCIETY. 



languished out an existence, — this supposition heing strengthened by 

 the fact, that the Irish localities recorded for it are adjacent to, or at 

 least in the same districts as those in which Polystichum Lo^ichitisj an 

 undoubted boreal species, is found (the late Professor Smith, in his de- 

 scription of the Flora of the Alps, vide ''Natural History Keview,'* 

 page 48, vol. iv., mentions his having remarked these ferns growing 

 abundantly in juxtaposition at a considerable elevation). 



Of the place of origin of Trichomanes radicans and Asplenium acutum 

 there can be little doubt, Lusitanian is stamped on every fact con- 

 nected with their distribution. 



This view of the origin of TVichomanes radicans enables us to re- 

 concile the seeming contradiction of its present and past distribution in 

 the British Isles ; for, looking at L. Fcenesecii as a member of the same 

 Flora, that this is so is evident, since the district in which it is at 

 present most characteristic and abundant also furnishes such confessedly 

 Lusitanian types as Erica ciliaris, E, Sihaldis, E. Mediterranea, Asple- 

 nium acutum J A. lanceolatum, Simethis bicolor, Pinguicula grandiflaray 

 &c., among plants; and among animals, Bufo calamita, Helix pisana^ 

 Anthrocera minos {?), Geomalacus maculosus, Echinus lividus, Cossonus 

 Tarda, Thia polita, «&c., — forms sufficiently characteristic to mark the 

 district as possessing a great Lusitanian colony, many of the species not 

 occurring elsewhere in Ireland and in England, only in the southern 

 counties. 



Giving due weight to the difference of habit of growth, — Tricho- 

 manes requiring three to four years for the full development of its 

 fructification, and Loph. Fcenesecii perfecting its spores in the course of 

 a season; bearing in mind the different physical conditions under which 

 these two species are capable of existing, — an examination of their pre- 

 sent distribution furnishes at once the clue as to how Trichomanes could 

 have ever occurred atBellbank (Yorkshire), and Powerscourt ( Wicklow). 

 The following general laws of distribution explain the whole diffi- 

 culty : — 



I. A species having spread from its centre, that disturbing causes in- 

 compatible with its existence may arise at any point in the track 

 of that distribution ; the range of its existence may cease at this 

 point, and the continuity of the distribution be destroyed, giving 

 rise to a colony or colonies. 



II. That this interruption of continuity may proceed even to the total 

 destruction of the capital centre, the colony or colonies remaining 

 in vigour, or at least existing subsequent to the destruction of the 

 capital. 



III. That the disturbing causes having been removed, and the range 

 of existence being thus restored, the species may spread from this 

 colonial centre or centres as from the capital, or even re-colonize 

 the capital. 



Let us examine in this point of view the recorded range of Trich. 

 radicans and Loph. Fcenesecii. Trichomanes flourishes in a district extend- 

 ing from about 51° SC N. to 52° 40' N. (Bantry being the most southern 



