EFFECT OF CHANGES IN THE VOLUME OF WATER. 325 



terraces on tlie border of certain salt lakes which once 

 formed a part of the Sea of Aral were found to greatly 

 vary. As the lakes gradually became dry the water grew 

 Salter, and this, with the diminished volume and the 

 increasing average temperature, probably resulted in caus- 

 ing the variations. 



Changes in the Volume of Water.— This has been found 

 to affect the size of shells, just as r)iankind in densely 

 crowded cities is more dwarfed than where country bred. 

 Semper separated specimens of Limnma stagiialis from 

 the same mass of eggs as soon as they were hatched, and 

 placed them simultaneously in bodies of water varying in 

 volume from 100 to 2000 cubic centimetres. All the other 

 conditions of life, food, etc., were kept at the known op- 

 timum. He found as the result that the size of the shell 

 varied directly in proportion to the volume of water in 

 which it lived, and that this was the case whether an in- 

 dividual specimen was kept alone in a given quantity of 

 water or shared it with several others (Fig. 336). 



a " 



4 ^ 



Fig. 33G. — Four equally old shells oF Liinvfrn sfarpinli^ hatched from the same 

 mass of egf^s, but reared in diffei-ent vohiiiips of watei-. o, in 100 cubic 

 centimetres; h, in 250 cc; c, in GOO cc. ; and d, in XJOOO cc. After Semper. 



I)e Varigny has greatly extended Semjier's experiments 

 on pond-snails (Limna?a), and concludes that in a lessened 

 volume of water dwarfago is due to the lack of exercise and 

 of movement, and that snails reared in small bottles become 

 more dwarfed the more they are crowded. The size of the 

 snails incrwised with the superficies of the water. The 

 conditions of volume and of superficies were identical, and 

 under both conditions the snails were smallest when they 

 were most crowded (Fig. 337). 



Influence of Light. — Our light comes from the sun, and 



