660 Proceedings of the Royal Irish Academy. 
In tlie foregoing list 421 species are dealt with. That is to say 
there are that number of plants in Ireland occurring somewhere or 
other at an altitude of 500 to 700 feet or upwards above sea level. ^ 
Theoretically, if we take a species which reaches that height only in 
the southern counties with a milder climate, it should not ascend 
above sea level in the northern counties at all. And further, a plant 
which reaches that height in the extreme north of Ireland should 
ascend to about double the altitude in the south. These observations 
relate to the lowland not the alpine species, that is to say, to those 
with an upper limit, but no lower, in Ireland. And it has been 
remarked already that the Irish alpine species indicate that this 
climatic difference between the south-west and north-east of Ireland 
is somewhere about 800 feet, perhaps 1000 in extreme cases. 
Let us see from the list whether any number of species from the 
plains follow this law. Thoroughly marsh-loving or aquatic species 
should not be used as test species since they are far more ubiquitous, 
in consequence of being less dependent on atmospheric effects than 
the rest.^ In selecting these examples it is not, of course, necessary 
to adhere only to those at or about the 600 feet height. What we are 
seeking for is the difference ; whether this be evidenced by the dis- 
appearance of the species altogether from the mountains in the north, 
or by whatever altitude it decreases or varies from south to north. 
From the diagram of alpines \_see Table], it will be seen, that 
taking Donegal as the base line, the mean alpine heights in the re- 
maining mountain districts increase in the following order : — X, 0 ; 
IX, 231; YIII, 377; YII, 1000; YI, 309; Y, 945; lY, 1100; 
III, 1190; II, 1080; I, 855. 
Of these, lY. (Blackstairs and Mount Leinster) hardly deserve con- 
sideration, so few are the species therefrom. At the same time it is 
my belief that, if there were mountains in that district containing 
alpine plants, the latter would occur at a higher level there than else- 
where in Ireland. 
^ There is, generally speaking, on any mountain a difference of about 200 or 
300 feet in the altitude to which lowland plants ascend on the north and south 
sides, the latter heing of course the highest. It was for this reason that my lower 
limit wavered between 500 and 700 feet. 
2 Thus in a lake in Donegal, Typha angustifolia, and Myriophyllum verticillatum 
grow side by side with Callitriche autumnalis, Isoetes lacustris, Sparganum natans, 
and Fotamogeton filiformis \ two groups of species whose juxtaposition would be 
quite unprecedented were it not for the equalizing influences of aquatic life. 
