38 GUERNSEY. 
have an unhealthy look about them, and do not fructify at all freely. 
Desmids are remarkably scarce, and give one the impression that as 
a family they are dying out. Repeated gatherings made at different 
seasons at Grande Mare and elsewhere have failed to produce 
even a dozen different Desmids, whereas on the Cornish moors 
it is nothing unusual to find thirty or forty species in a pool a 
few yards square. A single gathering which I once made on 
Tremethick Moor, near Penzance, contained no less than sixty- 
eight species of Desmids alone, besides other unicellular Algae. 
Doubtless the salinity of the water in stagnant pools accounts in part 
for the paucity of Fresh-water Algae in general, and particularly of 
Desmids ; but there must be some other cause besides. My own 
belief is that it is largely due to the natural drainage of highly 
manured land, and the pollution of cattle, and this view is supported 
by the generally feeble growth and unhealthy condition of the bulk 
of the specimens. 
Although strictly a subdivision of the Algae, it is customary to 
keep the Diatomaceae apart, and they will be so treated in the 
systematic portion of this work. A great deal still remains to be 
done before an exhaustive list can be compiled, but what has been 
done sufficiently shows how rich the island is in these microscopic 
forms of vegetable life. Mr. Thomas Rylands, an accomplished 
diatomist, who examined the material collected in Guernsey by 
Dr. Wallich in 1858 and 1859, and prepared a list of the species it 
contained, remarks : ‘It would be difficult to name a locality more 
likely to reward with success a diligent search for these things in 
their natural haunts than the shores of Guernsey, or in general to 
point to a more desirable field for the collection of Diatomaceae 
than the Channel Islands.’ 
No vestige of fossil Diatouis has ever been found in Guernsey. 
On several occasions I have prepared and carefully examined 
samples of various deposits selected for the purpose by geologists 
in the island, hoping to find traces of these organisms, but nothing 
of the kind has been detected. 
The vegetation of an island such as Guernsey may be divided 
into two sections: first, the truly indigenous plants, the aboriginal 
possessors of the ground; and, secondly, the exotic contingent, or 
introduced plants, those which did not originally belong to the 
place, although at present they are perfectly wild, and form an 
important part of the local flora. By far the largest number belong 
to the former category ; they are the old inhabitants of the soil, 
which have existed there for ages, long before the advent of man 
upon the scene. These alone, strictly speaking, constitute the 
natural flora of a district. But intermixed with these, and growing 
side by side with them, there are numbers of plants belonging to 
the second category, species not truly native, however much they 
