ON INFUSOltlA AND LYMN7EA. 109 



form ■will still be able to reproduce and multiply itself ; for the 

 maturation of germinal matter — the ovum and sperm — takes 

 place during the winter and early spring, at a time when the low 

 temperature of the water hinders all growth, and the optimum 

 of warmth for the sexual processes is much lower than that for 

 growth. Thus a permanently diminutive race '^ might arise if the 

 conditions of temperature above described remained constant for 

 several successive years in the lake or stream where the young 

 molluscs or the eggs have been deposited. Hence it has been 

 supposed, and in many cases no doubt with justice, that the 

 dwarfed races of animals which are found on high mountains or 

 in the polar regions, where they must meet with the conditions 

 of temperature just described, have originated directly from the 

 low temperature hindering their growth. This assumption, as 

 is quite evident, perfectly accords with my experiments on 

 Lymnsea ; still it must not be forgotten that other influences 

 might have precisely the same effect, such as insulficient food 

 and other circumstances, which will be discussed in the next 

 chapter. 



Thus we have seen that a degree of cold nearly as low as 

 the freezing-point of fresh v,rater kills Infusoria, but not the 

 Lymnsea ; also that their vital activity, as shown by the con- 

 traction of the pulsating vesicle, begins at 4° above zero, while 

 assimilation, and with it the other organic functions, only begin 

 in the Lymnsea at 12° above zero, although the optimum of 

 temperature which favours the highest vital activity and de- 

 velopment is almost identical for both, viz. 25°. The enormous 

 difference in the powers of resistance to a fall in temperature, 

 possessed by diflFerent creatures, which is proved by these ex- 

 amples (and by many others which need not be particularly 

 mentioned), shows that its influence is not absolute, i.e. cannot 

 equally affect all the auimals exposed to it ; but that the ex- 

 tinction of certain animals under secular refrigeration in either 

 hemisphere of the globe must have depended in part on the nature 

 of the animal itself. 



This reaction against the influence of cold is in the highest 

 degi'ee dissimilar in different species of the same family. A few 

 special phenomena illustrating this, particularly those of hyber- 



