ENVIRONMENTAL FACTORS 45 



many marine animals, and of groups like spiders on land. 

 Vallentin ^^^ states that on the Falkland Islands the waters are 

 acid owing to the peaty nature of the soil, and that in winter 

 this acid water runs down on to the shore and rots the shells of 

 the mussels {Mytilus) which grow in vast numbers in some 

 places. It is obvious that acidity is a limiting factor to an 

 animal whose shell is made of calcium carbonate. This 

 becomes an important factor with fresh -water molluscs . Waters 

 with a hydrogen-ion concentration of less than 6 seem to be 

 deficient in molluscs. But here again we are in danger of 

 only establishing a correlation, since such molluscs (as also the 

 crayfish) also require a certain minimum salt-content of water 

 in order to build up their shells properly. In these cases the 

 ideal thing is to combine very wide field observations with 

 laboratory experiments in which one factor is varied at a time, 

 the rest being kept constant. This was done by Saunders "^^ 

 in the case of the large protozoan Spirostomum ambiguutn. 

 Studies in the field showed that Spirostomum was most active 

 at a pH (hydrogen-ion concentration) of about 7-4, and 

 occurred most commonly in such habitats. This was 

 confirmed by experiments which showed that it died in 

 water of about pH 8 or at a pH below 6, the optimum 

 condition being 7-4. When placed in a gradient of pH it 

 migrated into the region of 7-4 (but for some reason it did 

 this only in the light, regardless, however, of the direction 

 from which the Hght was coming). This piece of work is an 

 almost ideal example of the way to set about the study of 

 limiting factors : field observation on occurrence and activity 

 followed by experiments on the limits of endurance of the 

 animal, the whole being clinched by putting the animal in a 

 one-factor gradient in order to see what its reactions were. 



14. MacGregor ^2 has worked out the effects of hydrogen- 

 ion concentration upon the distribution of certain mosquitoes 

 in the south of England. There is one species {Finlaya 

 geniculata) which lives as a larva in tree holes containing small 

 amounts of very acid water. The normal pH of these tree- 

 hole waters is well below 4-4. In experiments it was found 

 that the larva develops normally in jars of water whose pH 



