SEMPER'S EXPERIMENTS 79 



with little to eat. Of course it is quite natural that 

 organisms which do not eat become smaller, and if 

 under-fed as a rule they must doubtless remain of 

 inferior dimensions. But there are cases where, not- 

 withstanding abundant food, animals are unable 

 to grow to their accustomed dimensions. Herbert 

 Spencer says that it is well known by all anglers that 

 trout and other fishes are small in small streams, and 

 large in larger rivers, and many naturalists are of the 

 same opinion. Is it that these animals remain small 

 because they get less to eat ? or is there some other 

 reason ? 



I have also made some investigations on this 

 subject during the past two or three years, and may 

 be allowed to recall them. The starting-point of 

 these investigations was the fact announced by Karl 

 Semper some twenty years ago, in a special paper 

 on the matter, which he has since abstracted in his 

 Animal Life, that if the common pond snail is kept 

 in small volumes of water, of less than five or six 

 litres, the animal does not attain its regular develop- 

 ment, and remains more or less dwarfed. For instance, 

 if three young pond-snails (Lymncea stagnalis, or 

 L. auricular -ia), of the same brood and age, are 

 put respectively into aquaria containing 500, 1,000, 

 and 3,000 cubic centimetres of water, a difference 

 in their dimensions may be detected even after a 



UNJVEESIIl 



