SECTIONAL TRANSACTIONS.— D. 341 



I reached my first base, the camp of the East African Archaeological Expedition, 

 on April 16 to work Lake Nakuru and Lake Elmenteita. On June 5 I moved forty- 

 miles south to a friend's farm to work Lake Naivasha until July 11, when I had to 

 leave to catch the boat again on July 13. 



I was also able to visit Lake Baringo on May 2 and a small Crater Lake on June 16. 

 On Lake Nakuru (9' 3" max. depth) four days were spent in taking water samples, 

 reading temperatures and hydrogen-ion concentrations and collecting with the 

 townet and grab. Three other days were spent in making photosynthesis experiments 

 on the depth of penetration of light, using the filamentous green Algae that were 

 abundant in the lake. This lake was highly alkaUne, due, it is beUeved, to the presence 

 of sodium carbonate. 



On Lake Elmenteita (6' 2" max. depth) three days' collecting showed the lake to 

 be of the same type as Lake Nakuru, although slightly less alkaline (•22N as against 

 •27N) and supporting apparently two species of planktonic Rotifers instead of one. 



On Lake Naivasha (60' max. depth) nine days were spent in collecting at different 

 deep-water stations. The water proved to be comparatively fresh (•004N) and good 

 quantitative hauls of planktonic Copepods and Ostracods were obtained from a 

 number of depths. Four days were also spent here in attempting, rather unsuccess- 

 fully, to make photosynthesis experiments with Myriophyllum sp. A further three 

 days were spent in collecting among the inshore weeds, where definite zoning was 

 apparent, and rich catches of Cladocera, Ostracoda, Insecta, Hirudinea and Polyzoa 

 were obtained. Mollusca were present here though absent from all the other lakes 

 examined. Fish were also present in Lake Naivasha as well as in Lake Baringo, the 

 latter also having quite abundant planktonic Entomostraca, but no weed-inhabiting 

 fauna, since the alkaline content ('OIN) was apparently high enough to inhibit the 

 growth of macroscopic plants. 



The variety and abundance of the fauna appeared to vary in inverse proportion 

 to the quantity of alkali in solution in the waters of these lakes. 



The shallowness of the soda-lakes prevented the estabUshment of any very definite 

 la3'ering, except in the case of the oxygen content ; this was measurably lower in the 

 layers immediately overlying the decomposing mud on the floor and shores of these 

 lakes than in the layers nearer the surface. 



I was promised the use of a large motor-boat on Lake Naivasha, but unforeseen 

 circumstances prevented its being available till my last daj"-, when some good catches 

 were obtained from it. My scheme for taking samples at night had therefore to be 

 given up, as it was not considered wise to attempt it in the small rowing boat used by 

 day. 



Twelve days were spent in packing, unpacking and moving from one base to 

 another. 



Most of the remaining fifty-four days were spent in fixing and preserving the 

 specimens and carrying out analyses, on about forty water samples, for alkah-reserve, 

 dissolved oxygen and, in many cases also, for silicates, phosphates, chlorides and 

 magnesium, the different lakes showing a vnde range of variation. 



The specimens collected, including a number of MammaUan parasites, some insects 

 and a few snakes and hzards, &c., have all been sent to the British Museum for further 

 investigation. 



Dr. L. B. Ripley and Mr. G. A. Hepburn. — Olfactory and Visual Reactions 

 of the Natal Fruit-Fly {Pterandrus rosa) as applied to Control. 



Presidential Address by Prof. D. M. S. Watson, F.R.S., on Adaptation. 

 (Sse page 88.) 



Dr. C. M. YoNGE. — The Great Barrier Reef Exfedition. 



Afternoon. 



Visit to the Institute of Medical Research (by the kind invitation of 

 the Director). 



