344 SECTIOXAL TRANSACTIONS.— D, E. 



predaceoiis fish. Further ecological work will be done on Lakes Rudolf, Baringo and 

 Edward in 1930-31, and the object of the paper is to invite criticisms and suggestions 

 as to methods of research rather than to recount results from past work. 



Miss P. M. Je.vkix. — By examining the physical and chemical conditions in lakes 

 the ecological problem discussed by Mr. Worthington can be attacked from the other 

 end : and by measuring those conditions in conjunction with the distribution of the 

 smaller organisms, at different times and places, another chapter may be added to 

 the story told by the food-chains. In all environments there is a constant interchange 

 of chemical substances between the organisms therein and their environment, but 

 when this latter is watery, the estimation of the interchange can be most readily made. 



Many of the simpler chemical substances to be found in solution in fresh water 

 together form the food supply of the phytoplankton, and their absence may act as 

 limiting factors for growth. Such substances are present in quantities directly 

 dependent on the nature and amount of all the living and decaying organic matter in 

 the water. It should be possible to make accounts for phosphates or carbon dioxide, 

 for instance, balance. 



But there are other factors influencing the organisms in a lake, by altering the 

 conditions under which they live. Of these the two most important are : — 



1. Clitnatic factors changing the physical conditions and having important effects, 

 both direct and indirect, upon fauna as well as flora. In extreme cases these, too, may 

 act as limiting factors. 



2. Geological and Topographical factors affecting the chemical conditions in the 

 Lakes by the supply and withdrawal of water and salts in solution. The Rift Valley 

 Lakes fn Kenya, for instance, have no outlet, and the consequent concentration of 

 soda, dissolved out from surrounding alkaline lavas, varies so widely (L. Naivasha, 

 0-004 N. alkali reserve, to L. Nakuru, 0-29 N.) as to be the apparent cause of sharply 

 marked differences in fauna and flora in the series of five lakes examined. If this 

 apparent correlation be true the problem arises as to which of the links in the many 

 food-chains may be directly influenced by such a factor, and which only indirectly. 



The work on the Rift Valley Lakes is unfinished and is only referred to in the hope 

 of raising suggestions and criticism on methods of further investigation. It seems 

 to indicate the possibility of finding, in Africa, series of lakes varying quantitatively 

 in one or two fundamental factors only, and offering a line of investigation into the 

 relation of the environment to the lowest and most important members of the food- 

 chains. 



SECTION E.— GEOGRAPHY. 



Thursday, September 4. 



Mr. W. W. Jervis. — General Introductory Survey of the Bristol Region. 



Mrs. D. PoRTWAY DoBSON. — The Bristol District in the Prehistoric Period. 



The simplest way to explain the occupation of the Bristol district during the 

 prehistoric period is by means of distribution maps. Th*e shew that in lower Palae- 

 oKthic times the district was extensively occupied, for animal remains are found in the 

 caves and fissures of the limestone rocks, and also in the Pleistocene gravels of the 

 river valleys. Human occupation in the pre-Mousterian times is only proved, so far, 

 for some of the gravels. 



In upper PalaeoUthic times the distribution is in some respects the same, as certain 

 caves contain the bones of animals belonging to the fauna of both early and late 

 Pleistocene times. The newer loess, however, also shows traces of animal life belonging 

 to upper Palseolithic times, as do certain cave sites such as Avehnes Hole, wliich have 

 yielded no bones of the older fauna. Artifacts probably of Mousterian times have 

 been found, and a great quantity of flint implements and other traces of Aurignacian 

 Man have been recovered from the Mendip caves. 



Of Neolithic times there is at present scant trace, save in two caves, but of the 

 Megalithic period, or the time of transition from the stone to the true Bronze age, there 

 is abundant material, illustrated both by megalithic structures and beaker pottery. 

 Generally speaking, this comes from the high or moderately high ground, with a few 

 notable exceptions. 



