Certain Laws of Variation. 



89 



an additional 4 days at 13*3 they would doubtless have grown some- 

 what more. The increase for the 1st to 84th hours is therefore some- 

 what more than this 9 '63 per cent., though less than 11 '98 per cent. 

 Let us take it as 10'80 per cent., or 0'130 per cent, per hour. Again we 

 see that the larvae kept 8 days at 20'3 are only 2*35 per cent, larger 

 than those kept 3i days at this temperature. Hence the maximum 

 effect capable of being produced by the more favourable temperature 

 during this 84th to 192nd hour must be somewhat less than 0*022 per- 

 cent, per hour, or not a fifth of that produced in the earlier period. 



The next experiment was made in July. After an hour's impregna- 

 tion at 22 '7, some of the ova were poured into a jar of water which 

 stood in another jar which was surrounded by water and ice. By this 

 means the developing ova were kept at about 12. Every few hours the 

 water was stirred up and portions of it poiired into jars, which were then 

 transferred to the tank water. This had a mean temperature of 22-5. 

 The following results were obtained : 



Here we see that the effect produced during the 1st to 6th hours was 

 four times as great as that during the 6th to 10th hours. In another 

 experiment some ova, which had been impregnated at 1 C., and had 

 thereby given rise to larvae 3*6 per cent, smaller than the normal, were 

 kept for the next eight hours at 6. The larvae resulting therefrom 

 were still 9*34 per cent, smaller, or, on an average, were diminished 

 1*17 per cent, for each hour of exposure to the abnormal temperature. 



As it was found somewhat troublesome to keep a considerable volume 

 of water some twelve or fifteen degrees below that of the atmosphere 

 for many hours, the rest of the observations were made on the effects 

 of keeping the developing ova at a higher temperature than the normal. 

 As the temperature of the air in the summer months at Naples, where 

 these experiments were made, varies but little from day to day, it was 

 easy to keep the water in a tank holding about 30 litres at a practically 

 constant temperature throughout the experiment. In fact, it did not 

 vary more than 0'3 or - 5 at the most. In such a tank, if left un- 

 covered, the temperature of the water was found to fall by evaporation 

 to about 25, or about 2 lower than that of the atmospheric tempera- 

 ture. By covering it up, this could be diminished if wished, and small 



