SECTIONAL TRANSACTIONS.— D. 373 



Tuesday, September 12. 



19. Dr. W. R. G. Atkins.— 77ie Hydrogen ion Concentration of Soils 



and Natural Waters in Relation to Animal Distribution. 



It lias been shown that a close connection exists between the distribution of 

 plants and the soil reaction. Observations prove the relation of many animals, 

 especially insects, to specific plants. Their distribution is, therefore, indirectly 

 related to the soil reaction. A direct relation also exists. Certain earthworms 

 are poisoned by the peaty soil of Dartmoor, and 0. Arrhenius has shown that 

 earthworms die outside certain limits of reaction. Snails, too, are similarly 

 affected, HeiiccUa caperata and //. virgata have never been found on acid soil. 

 Among Crustacea, Asellus mpiaticus lives in slightly acid water and Gammarus 

 jmkx in alkaline streams. In captivity G. jm/rx lives and breeds only if in 

 water alkaline from calcium salts. ^lacGregor has shown how important tlie 

 reaction of the water is for mosquito larvse. 



20. Miss K. Carpenter.— Fres/i Water Fauna of Aberystwyth Area 



in Relation to Lead Pollution. 



Faunistic study of the streams of this area reveals a general paucity referable 

 to the scarcity of lime salts, but mure remarkable is the almost complete barren- 

 ness of the larger rivers, Rheidol and Ystwyth. Out of a total of sixty Inverte- 

 brate species (Protozoa, Rotifera and Nematoda e.xcepted) collected in local 

 streams, only eleven occur in the main stream of the Eheidol and nine in the 

 Ystwyth; further, these waters contain no fish. The Clarach, a smaller stream 

 (length eight miles, Kheidol twenty-five, Ystwyth twenty-six and three-quarters), 

 has nevertheless a fauna of thirty-eight Invertebrate species at one locality, and 

 also contains trout. These last occur in several quite small brooks, many of 

 which have a considerably longer fauna-list than the main rivers. The selective 

 factor is undoubtedly the pollution of the rivers by contamination from mine- 

 waters and refuse-heaps on the sites of old lead-workings in the hill regions; 

 correlation of fauna-lists with chemical analyses of water and sediments 

 establishes this beyond a doubt, but there remains the further question of the 

 actual method of operation of this factor, whether through clogging of delicate 

 organs by galena-grit or through infiltration of dissolved lead-compounds. On 

 the former and generally favoured alternative are based the recommendations 

 of the Ministry of Agriculture and Fisheries for safeguarding river fisheries 

 against lead pollution, yet there is strong evidence of the working of the second 

 factor in this district. The Clarach, like the Rheidol and Yst\vyth, has mines 

 near its headwaters, and the percentage of lead solid in river sediment is 

 highest in the Clarach (0.12 per cent. Clarach, 0.085 per cent. Rheidol, 0.01 per 

 cent. Ystwyth, in normal times), so that the compai'ative wealth of species in 

 the river already noted can only be related to the absence of any appreciable 

 quantity of dissolved lead substance, in contrast to the normal proportions of 

 0.02 and 0.04 mgr, per litre in Rheidol and Ystwyth respectively. Even if 

 mechanical clogging of the gills occurs in fishes, recent observations definitely 

 point to the solution factor as affecting the Invertebrate food organisms and the 

 supporting flora. Further evidence of similar trend has been derived from 

 observation of the peculiar conditions consequent upon last summer's drought. 



21. Mr. Julian S. Huxley. — Titne-Relations in Amphibian Meta- 



morphosis. 



Metamorphosis represents the point at which one phase changes into a second. 

 The species in metamorphosing organisms is dimorphic, but the dimorphism is 

 consecutive. An interesting parallel can be drawn with sexual dimorphism. 

 In most animals sexual dimorphism is simultaneous, but in protandric and 

 protogynous hermaphrodites it is consecutive. It is also consecutive in the 

 ' intersexes ' experimentally produced by Goldschmidt by crossing geographical 

 varieties of Lymantria, and found in other forms by other workers. Consecutive 

 sexual dimorphism is thus accompanied by what may be called a ' sex meta- 

 morphosis.' In all examples of consecutive dimorphism, whether the meta- 

 morphosis is from larva to adult or from one sex to the other, the morphologicai 



