64 SECTIONAL ADDRESSES 
these communities. Such work certainly justifies the recognition of 
associated species, a pond community differing in its composition from 
that of a lake or river, and a further study of ponds, lakes and rivers shows 
that they also can be classified into types, in each of which the community 
of water-beetles differs in species-composition from the others. A silt- 
pond, for instance, in clay or marl, gravel or sand, has a community 
differing markedly from a detritus pond, where decaying vegetation has 
produced a muddy bottom and somewhat turbid water. Again, a brackish 
pond provides the collector with species not to be found in the previously 
mentioned types, and the peat pond produces other species. The silt 
pond is by far the richest in species and, out of about 250 in the British 
list, from forty to sixty species occur in it and may even be taken in one 
pond at the same time. 
It is, however, obvious that this classification of types of ponds, etc., 
is only a rough one, because the silt pond gradually changes to the detritus 
pond and all ponds are slowly changing their character to pass through 
a swampy marsh stage to dry land. The lake, around its edges, acquires 
pond conditions and the evolution of the lake follows that of the pond. 
Rivers are slowly changing, the less rapid parts tending to silt up and pro- 
duce pond conditions so that, although, as I have said, the intensive study 
of water-beetles justifies the recognition of groups of species associated 
with types of habitat, the same spot visited throughout a series of years 
will provide an ever-changing habitat and an ever-changing community 
of water-beetles. 
Now the obvious explanation of this community grouping of species 
is that some are less well adapted to a particular type of habitat and are, 
therefore, excluded from it by better adapted species, but is this the 
whole explanation? That there is a struggle for existence is beyond 
dispute, but is it as severe and determinate as is usually supposed ? 
Among vegetarian insects there is often a range of food plants, upon any 
of which they may be found. In most cases there is no evidence that the 
different plants carry monophagous races, in fact it is normal for such 
insects to remain healthy and to complete their life-histories when changed 
from one to another of the plants. In the case of some insects, either 
monophagous or oligophagous, other plants are resorted to under some 
circumstances, indicating that the normal food plants are the ‘ choice’? 
of these insects and that other plants are food reserves. The Small 
Eggar Moth ’ has a variety of food plants, but I have reared it successfully 
upon laurel, birch, elm, and some other plants which are not included in 
its normal menu. Among the bark-beetles, the majority commonly 
breed in dying or fallen trees or in trees which, owing to fire or other — 
cause, have been weakened. ‘These beetles are, therefore, in the nature 
of scavengers, clearing away dying trees ; but at times when great damage 
has been done to the forest by wind, fire or snow, they increase greatly 
in numbers and then attack not only healthy trees of their normal food 
2 It will be understood that I am using the term ‘ choice’ in the same sense 
as a scientific man uses the term ‘ free will,’ knowing that every action is an 
effect following a cause. 
3 Eviogaster lanestris, L. 
