66 SECTIONAL ADDRESSES 
when another species is safe from attack, it is not reasonable to explain 
these more or less constant communities in that way. Moreover, unless 
we assume that the ‘ purity’ of a community is maintained by the 
immediate destruction of all emigrants, the struggle for existence cannot 
account for the fact that at no time of the year is there a general mix up 
of communities, even when the new generation, overcrowding its birth- 
place, is migrating elsewhere, indicating that the emigrants choose their 
new homes and do not go to the first water they may find. Ponds in areas 
liable to flooding may contain river species after the flood has subsided, 
but such species will be rare or absent after a period free from flood. 
Neighbouring ponds in an area where ponds are numerous, such as 
brick pits, often provide certain species in certain ponds, even for periods 
of two or more seasons, although the general fauna of all the ponds may 
be otherwise similar. In many places in Scotland the roads in the 
moorland districts are repaired by digging the boulder clay along the sides 
and using this material as ballast. ‘These holes quickly fill with water 
and form typical silt ponds. In the course of time, sphagnum and other 
vegetation fills them and they become peat pools. In Lewis and Harris, 
in 1914, I paid special attention to these ballast ponds. Of the 23 
examined, 15 were either free from any weed or contained water-grass 
(Glyceria) with, at most, traces of sphagnum ; that is, they were recently 
dug holes. In the 15 ponds 30 species occurred, out of the 52 taken 
by me in the island. Of these 30 species, 13 occurred proportionately 7° 
more often in these ponds than elsewhere, indicating that these species, 
or at least some of them, sought out these silt ponds in preference to the 
much more numerous bog pools. In one of my aquaria, about 12 ft. long 
and 2 ft. wide, there is a bank of stones and earth dividing it into two 
ponds. One of these is preferred by some of these species of water-beetles 
and they always migrate to it when placed in the other. The one they 
prefer is also preferred by the water-shrimp (Gammarus) which is always 
more abundant in it than in the other. 
All these examples show that choice plays a part in the composition 
of these communities, but, just as water-beetles are grouped into com- 
munities associated with types of habitat, so also can they be allocated 
to groups according to their distribution. In the British Islands these 
groups have been recognised, not only for the water-beetles but for the 
whole fauna and flora. H. C. Watson,!! in 1832, divided the country 
into provinces based upon the groupings of the plants, and in 1846, 
Edward Forbes !2 showed that similar groupings could be made of the 
animals. Forbes was the first to offer an explanation and he regarded 
10 The method of determining the proportional occurrence of these species is 
as follows: Hydvoporus nigrita, Fab. occurred in 4 of these 15 ponds and in 
14 of the other 115 collections. Fifteen is about one-eighth of 130, so that, if 
the species were generally dispersed, it should have occurred in 28 of the other 
collections. Actually it only occurred in 18, so that, proportionately, it was 
more common in silt ponds than elsewhere. 
11 Outlines of the Geographical Distribution of British Plants, Edinburgh, 1832, 
and The Geographical Distribution of British Plants, London, 1843. 
12 ‘ The Geological Relations of the Fauna and Flora of the British Islands, 
etc.,’ Mem. Geol. Survey, vol. i, 1846. 
