CERTAIN LAWS OF VARIATION. 193 



mentation can scarcely have begun; hence it must be 

 the temperature of the ovum and the spermatozoon 

 during the act of impregnation which is of such con- 

 siderable influence on the subsequently developing 

 larvae, the effect perhaps depending on the vibratile 

 energy with which the spermatozoon strikes and enters 

 the ovum. In the last line of the above table are re- 

 corded a few experiments in which the time of exposure 

 to the abnormal temperature was reduced to ten sec- 

 onds. The average effect then produced was only 1.7 

 per cent., or less than half as much as in the other ob- 

 servations. Presumably, therefore, the time was in- 

 sufficient for all the ova to become impregnated at the 

 abnormal temperature. 



It may perhaps be thought that this extraordinary 

 sensitiveness of the ova to temperature at the time of 

 impregnation is true only for the particular species of 

 sea-urchin worked with, viz., Strongylocentrotus lividus, 

 and that one is in no way justified in regarding it as a 

 general phenomenon. How far this criticism is true or 

 not, future experiment alone can show, but the sensi- 

 tiveness was at least demonstrated in one other case, 

 viz., the ova of the sea-urchin Sphcer echinus granularis. 

 Exposure of the ova for one hour to a temperature of 

 about 11 produced in four experiments respectively 

 7.7, 1.1, 4.6 and 2.6 per cent, diminution in the size of 

 the eight-days' larvae. Exposure to a temperature of 

 27 was much less effectual, it producing in two experi- 

 ments respectively 2.5 and .9 per cent, diminution. In 

 two other experiments, exposure to this temperature 

 for a minute instead of an hour produced respectively 

 4.3 and .9 per cent, diminution. In explanation of 



