SECTIONAL TRANSACTIONS,—D. 487 
them. In this connection the group of insect parasites known as parasitoids 
(‘ refined predators ’) are very important and haye been used in many parts 
of the world to limit the numbers of noxious organisms. The problems 
involved in the use of these beneficial insects are obviously problems of 
population, since they deal with the numerical interaction of the parasites 
and their hosts ; but they are also problems of animal psychology, since the 
behaviour of the individual parasites and hosts must be considered. 
One of these problems is that of the selection of suitable hosts by the 
parasite. It has been supposed that parasites are distributed in a given 
host population at random, and on this assumption a theory of parasite dis- 
tribution has been developed. But the field observations and the laboratory 
experiments described show that the basic assumption is not true. Some 
parasites have the ability to discriminate between suitable hosts and those 
that have already been parasitised, and they distribute themselves much 
more efficiently among the host population than chance would do. This 
result is discussed from the two points of view, of populations and of insect 
behaviour. 
o 
AFTERNOON. 
Dr. G. C. Rosson.—The limitations of adaptability in the animal kingdom 
(2085). 
The main phyla and the subordinate groups of the animal kingdom differ 
markedly in their range of adaptability (i.e. specialisation for a particular 
mode of life). Differences of this kind are probably to be found in all 
animal activities, e.g. in the incidence of parasitism, methods of getting 
food, ete. The author dealt chiefly with the question of habitat-occupation, 
and pointed out that, whereas several of the main animal groups had been 
highly successful in colonising a large range of habitats, other equally large 
groups have a far more limited range. Thus the Insecta have failed to 
produce more than a few truly marine forms ; the Cephalopods have not 
established themselves in water below normal marine salinity ; the Echino- 
derms have not made their way into fresh water, though a few occur in 
estuaries. ‘The incidence of habitat-occupation in the various families of 
Gastropods was described and sundry cases of apparently idiosyncratic 
adaptability were given. The adaptive achievements of certain groups were 
then contrasted with their limitations, and the failure of the Lamellibranchs 
to establish themselves on land and of the Cephalopods to colonise either 
land or water of less than normal marine salinity was discussed. The 
nature and origin of the factors limiting adaptation were considered. The 
writer felt that individual predilection could not be excluded from the 
various factors determining habitat-occupation. 
_ Mr. A. Rorsuck.—The rook in the rural economy of the Midlands (3.0). 
Rooks have been studied in five midland counties—Leicestershire, 
Derbyshire, Nottinghamshire, Lincolnshire, and Rutland. 
A census of the whole area, repeated after an interval of four years, showed 
the rook population to be constant. 
About ro per cent. of the rookeries change their sites annually, 
__ There is no evidence that migration materially affects the problem in 
the Midlands. 
__ The distribution of their roosts during the winter is considerably different 
from that of their nesting rookeries, although many remain on the same 
sites all the year round. 
