102 



DUBLIN NATX7BAL HI8T0EY SOCIETY. 



Suppose two plants, species A and B, having the following range of 

 existence and standard of growth, — 



Now it is manifest these two species, though capable of existing in 

 the same district, are not uniformly so, but that one can exist in a dis- 

 trict where the other must perish. 



For instance, in a country whose average climate was represented by 

 the following range, L, 80° - 10 ; H, 80° - 25 ; and M, 90° - 10 ; A would 

 perish, except from a few favoured spots, and a slight further diminu- 

 tion of the standard would destroy it altogether ; while B would suffer 

 but slight inconvenience, but would be probably found least frequent in 

 the very station most favourable to A. Substitute for A and B in the 

 above, Trichomanes radicans and LopJiodium Fcenesecii, and the reason 

 of the persistence of the latter in districts in which the former has become 

 extinct is e\ddent, particularly if we remember the peculiar conditions, 

 as regards light and moisture, requisite for the well-being of Trichomanes, 

 and the perfect carelessness of Loph. Foenesecii as regards these. It but 

 needs the supposition of the destruction of the woods of a district by an 

 elevation of the temperature, or any other cause; the light would at 

 once mount up to a standard incompatible with the well-being of Tricho- 

 manes, which would then necessarily perish from every place, except a 

 few limited localities, where sufficient shelter might be afforded it by 

 rocky crannies, such as the Powerscourt station might afford, and here 

 the species wonld languish out a feeble existence, while Loph. Foenesecii, 

 heedless of the increased light, would still flourish on. 



One of the animal types, already mentioned, affords such strong cor- 

 roboration of this theory that I cannot resist quoting it. Helix pisana, 

 a Mollusc of South Europe, was found by Mr. W. Andrews at Iveragh, 

 Kerry. Next it occurs in a lengthened strip along the coast of Dublin, 

 Meath, and Louth, even as far south as Rush. Next it occurs at Tenby, 

 about half a degree to the south of its Kerry station ; and lastly, it is 

 recorded from Cornwall, that favoured spot, where still flourish many 

 confessedly Lusitanian forms. Let any one take the trouble of exa- 

 mining on the map the connexion between these localities, and compare 

 with them the distribution already laid down for Trichomanes and Lo- 

 phodium Foenesecii, and he will perceive that the connexion between the 

 two districts is too strongly marked to be merely accidental, but rather 



