EASE OF GKOWTH. 



163 



a period, lasting about three weeks, of very rapid growth, 

 during which it attaiiied a length of shell of about 10'2 

 millimfetres; in the third period — 40th to the 60th day — 

 it grew only 6 millimetres more ; and then ensued a period of 

 very slow growth, only about 1-6 millimetre in nearly four 

 weeks. It is thus proved that the same law obtains for 

 Lymnrea as for all other animals, namely, that the maximum of 

 growth can only be attained, when all the other conditions are 

 equally favourable, exactly within the period of quickest growth ; 

 the opportunity once missed never i;ecurs. A Lymnaea which 

 in the course of the first year has not attained a length of shell 



in days. 



-Fig. 45. — Curve of time for the growth of Lymnma slaanaliR. The growth is most 

 rapid from the foarth to the fifth week. 



of at least 20 millimetres must become the parent of a race of 

 dwarfs, if the causes which have checked its growth are regu- 

 larly repeated in the succeeding years. 



A direct influence is hereby proved to be exerted by the 

 volume of the water, irrespective of the supply of food and 

 other influences."' A short discussion of the details will show 

 whether we are as yet in a position to determine the special 

 causes of this effect of volume. 



It is self-evident that a great variety of influences might 

 have produced the same I'esult— ■i.e. the dwarfing of the animals 

 —such as food, temperature, injurious gases, or the absence of 



