THE CAUSES OF VARIATION. 237 



with 500 per litre; and with 17,500 per Htre they are 

 respectively 25*9 and 23*3 per cent, shorter. Variations in 

 the temperature of the water during development also pro- 

 duce opposite effects in the two cases. As we have already 

 seen, the body lengths of larvae allowed to develop at tem- 

 peratures above 22° are about 2 per cent, shorter than of 

 those at about 20°, but the aboral and oral arms are respec- 

 tively iO"8 per cent, and 8'5 per cent, longer. Again, whilst 

 the body length of larvae grown in diluted water is on an 

 average increased by 9-1 per cent., the arm lengths are 

 decreased by 77 and io'5 per cent. ; or, as the arm length 

 values are calculated as percentages on the body lengths, 

 they are practically not affected at all. Even the arm 

 lengths are not affected to the same extent by the same 

 change of environmental condition. Thus the ratio between 

 the arm lengths of larvae grown at temperatures above 

 22° is 4'3 per cent, higher than of those at temperatures 

 below 18°. 



It is unnecessary to make remark upon the experiments 

 made in respect of other conditions of environment, but a 

 passing notice may be given to the observations which 

 showed that the variability or amount of variation in the 

 size of the larvae may be affected by changes in the environ- 

 ment, as well as their actual average size. Thus the 

 dwarfed larvae obtained in the summer months varied in 

 size some 10 per cent, more than those obtained in the 

 spring. Again, larvae allowed to develop at 19° varied 18 

 per cent, more than those allowed to develop at 17", and 10 

 per cent, more than those at 23°. It would seem therefore 

 that the variation in an organism, whether it be in degree 

 or in the average size, with reference to a single organ or 

 to the correlation of organs, is equally capable of being 

 affected by changes in the conditions of environment. 



This conclusion is fully supported by the results of the 

 measurements made by Weldon upon shrimps and crabs (5). 

 By measuring some hundreds of individuals, and taking 

 mean values, Weldon showed that the specimens obtained 

 from various sources did not by any means correspond in 

 respect of the size and relation of their parts, but that they 



