162 



THE INFLUENCE OF INANIMATE SUEEODNDINGS. 



shells corresponding to this curve. The first of the shells, formed 

 in 100 cuhic centimetres of water, attained a length of only 6 

 millimetres; the second, in 250 cubic centimetres, was 9 milli- 

 metres long; the third, in 600 cubic centimetres, w,is 12 

 millimetres ; finally the fourth grew to 18 millimeti'es in 2,000 

 cubic centimetres of water. It scarcely need be repeated 

 that these animals, with such immense differences in length, were 

 all the oflspriiig of one mass of eggs simultaneously tiansferred, 

 and had all reached the same age of sixty-five days. 



My experiments also allowed of my constructing a curve of 

 time for the rate of growth of the Lymnsea. The reader may 

 have observed, with reference to the foregoing statements, 

 that according to this volume-curve it ought to be possible to 



200 400 600 800 1000 1200 1400 1600 1800 2000 cubic cen- 

 timetres of water. 

 Fig. 44. — Volume- curve for Lymncsa stagnalis. 



enable a Lymnsea to attain its fall length of about 24 milli- 

 metres (for the first year's growth) even in a volume of 100 

 cubic centimetres, if only it were left there for a longer time 

 than was requisite for acquiring that length in 2,000 cubic 

 centimetres. Still, this would only be po.ssible if the rate of 

 growth, as determined by the volume of water, were at all times 

 equal. This, however, is not the case. At first the growth is 

 very slow ; then succeeds a period of quickest growth, until the 

 oldei the animal is, the more slowly it grows. The curve ex- 

 hibited m the subjoined woodcut (fig. 45) was constructed from 

 experiments in a volume of water of from 1,000 to 2,000 cubic 

 cecitimeti-es per individual, and it shows that, during the first 

 three weeks after escaping from the egg, the growth of the young 

 animal was, on an average, only 5 millimetres ; then followed 



