186 
PROCEEDINGS OF SOCIETIES. 
Suppose two plants, species A and B, having the following range of 
existence and standard of growth ,— 
Range of Existence. 
Standard of Growth. 
Light, . . . 
Heat, . . . 
Moisture, . . 
A. 
50°—10° 
120 - 30 
200 - 70 
B. 
90°— 20° 
120 - 10 
170 - 10 
A. 
40°— 20° 
100 - 70 
150 - 80 
B. 
70°— 30° 
100 - 30 
110 - 20 
Now it is manifest these two species, though capable of existing in 
the same district, are not uniformly so, hut thatyme can exist in a dis¬ 
trict where the other must perish. 
Eor 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 Lophodium Fcenesecii, and the reason 
of the persistence of the latter in districts in which the former has become 
extinct is evident, 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. Fcenesecii 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 would languish out a feeble existence, while Loph. Fcenesecii , 
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 Fcenesecii , and he will perceive that the connexion between the 
two districts is too strongly marked to be merely accidental, but rather 
