Perthshire in Relation to ‘ Age and Area\ 325 
S 
The c Age and Area ’ hypothesis is so well known that it is here 
unnecessary to do more than point out that at least certain of the results 
obtained above agree perfectly with the general aspects of the theory. 
The widely distributed species in Perthshire prove to be common species 
in Britain. In both the rarity is 2*2. Chiefly Lowlan dplants show a 
higher degree of rarity both in Perthshire and in Britain, and these, taken 
together with the widely-distributed species (i. e. 502 species out of a total 
of 738), exhibit a remarkably striking descending series, quite comparable 
with the £ wides 5 of Willis. But Perthshire possesses 103 rare, entirely 
Lowland species, as many as 40 of these being limited to a single district. 
The numbers run in the opposite direction from the ‘wides’. Since these 
species are not endemics, their great rarity cannot be due to recent origin, 
but, on the view of ‘ Age and Area * to recent arrival within the county* 
They are confined very largely to Gowrie and Lowland Earn, where the 
bulk of the Perthshire flora seems to have entered, and they are thus 
possibly at an early stage of invasion. 
These three groups, widely distributed, chiefly Lowland, and entirely 
Lowland elements, form the bulk (605 species, or 82 per cent.) of the county 
flora. They constitute an assemblage of temperate plants and their mass 
distribution suggests migration from the east, since this temperate flora is 
concentrated in the Lowlands, as many as 553 species occurring in the 
small area of Gowrie, while the average number for the five Lowland 
districts is 518, or 86 per cent. In the Highland region, Highland Isla 
possesses the largest number, 454 species, but the average number in the 
Highland districts is 380, or 63 per cent., only. There is thus indicated 
a pronounced thinning out of the temperate flora towards the interior and 
mountainous parts of the county, although it is true that many species have 
spread often far into the Highland valleys. In general, then, this portion 
of the county flora affords further evidence in favour of ‘ Age and Area 
although the occurrence of an entirely Lowland group, of very marked 
rarity, may mean that other more vital factors result in certain species 
lagging behind the steady march of the main body. This is, of course, 
admitted, and will be due, generally, to ecological barriers, into the details 
of which in Perthshire we cannot here enter. 
Another aspect of the problem arises when the Highland element of 
the flora is considered. This is largely composed of species which are the 
recognized arctic-alpine or, at least, boreal members of the British flora. 
There is no evidence from the facts of present distribution that these plants 
follow the law of ‘ Age and Area ’. Nor shall we seek to apply it, since 
if the Highland species have a high degree of rarity, it is not because they 
are, in general, of recent introduction. We know from geological evidence 
that arctic-alpine species were formerly more widespread, an arctic flora 
occurring not only generally throughout Scotland in early post-glacial 
