164 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



those that were necessary, &c. In some of my experiments 

 such influences showed themselves very conspicuously. In one, 

 for instance, in order to supply the young animals with the 

 maximum of air required for respiration, I kept tip a constant 

 current in the vessel ; but the experiment failed totally, 

 because the young animals in the current were unable to cling 

 to the plants they fed on. On another occasion a sudden and 

 very considerable fall of temperature set in, almost down to 

 13 centigrade. Now this is the degree at which the powers 

 of assimilation of the young Lymnsea cease, and consequently 

 its growth. The effect on the curve of volume was very 

 striking (fig. 46). The vessels, of unequal size and containing 

 unequal bodies of water, stood side by side at the same distance 







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

 metres of water. 



FIG. 46. A curve of growth totally altered by a change of temperature. It continues to 

 rise as usual up to 500 cubic centimetres of water. There it suddenly falls, because the 

 temperature of the large body of water is insufficient to allow the Lymnaeidas living 

 in it to assimilate. 



from a window, where the sun shone in the afternoon for two 

 hours at the most ; thus the temperature favourable to assimi- 

 lation was attained in the smaller vessels, but not in the 

 larger ones. Hence it followed that snails which lived in 2,000 

 cubic centimetres of water, and consequently ought already to 

 have been 10 millimetres long when 25 days old, were 

 scarcely longer about 3 millimetres -than those which had 

 lived in water which, though less in volume, was sufficiently 

 warm. At the same time the nourishment provided in each 

 vessel was so abundant and wholesome that neither bad air nor 

 a lack of food could have occasioned the striking break in the 

 normal volume-curve; besides, in that case the effects must 

 have been visible in those in the small as well as those in the 

 larger body of water. 



