332 SECTIONAL TRANSACTIONS.— D. 



should be recognised : ultrabasic, basic, and acid. From each of these a 

 wide variety of rock-types can be produced by additive and subtractive 

 processes of differentiation. Further possibilities of variation are intro- 

 duced by mixing of magmas, assimilation, and differential fusion. Magmas 

 generated under stress are likely to be abnormal as compared with those due 

 to passive refusion. 



SECTION D.— ZOOLOGY. 



Thursday, September 1. 



Presidential Address by the Rt. Hon. Lord Rothschild, F.R.S., on 

 The pioneer work of the Systematist. (See p. 89.) 



Mr. A. J. Woodcock. — Notes on the natural history of the York district. 

 (See Scientific Survey of York and District.) 



Mr. A. Roebuck.— TAe numbers and distribution of rooks in the midland 

 counties. 



The proper study of a species must include its abundance and distribution, 

 especially if its economic position is to be determined. 



The rook (Corvus f. frugilegus) is a colony nester with, usually, con- 

 spicuous rookeries. This study has been made in an attempt to ascertain 

 the factors which influence the distribution and numbers of this species, 

 by taking a sufficiently large area. The area surveyed covers 5,305 square 

 miles and includes the five geographical counties of Lincolnshire, Notting- 

 hamshire, Derbyshire, Leicestershire and Rutland. 



It is impossible to define a rookery, as it may consist of a single nest, a 

 series of disconnected small groups, or a large compact mass. 



Any kind of tree, sound or unsound, may be used, and of any height. 

 The site may be in a very sheltered dell or on an exposed hill-top. 



The survey deals with 1,421 rookeries, containing 128,266 birds. The 

 average size of a rookery is 45 nests. The amount of food consumed is 

 over 4,000 tons per annum. 



Factors which influence their distribution are : available food supply, local 

 topography as influenced by the geological formation etc., presence of rivers, 

 abundance of grassland, and their relations with mankind. 



Dr. G. S. Carter. — Iodine compounds and the level of activity of animal 

 tissues. 



Variations in the level of activity of cells and tissues from time to time 

 during their life occur throughout the animal kingdom and in phenomena 

 of many different types. The activation of the egg at fertilisation and of 

 the sperm in the presence of the egg are examples of these changes, and 

 so also are the seasonal and diurnal variations of activity which result in 

 hibernation and daily sleep. The experiments to be reported refer to 

 these examples, but many others could be given. 



Experiments on the eggs and sperm of Echinoderms have shown that 

 substances chemically related to thyroxine play an essential part in the 

 manner in which the changes in the level of activity associated with fertilisa- 

 tion are brought about, and some experiments on the sperm of the rabbit 



