THE CAUSES OF VARIATION. 235 



found that, for instance, between Bermuda and New York 

 the specific gravity of the sea water varied from i "02504 to 

 1*02723 at 1 5 '6°, or enough to cause a difference of 13 per 

 cent, in the size of larvae developed under the extreme 

 conditions. The salinity of the waters of the Bay of 

 Naples, especially near the coast, probably shows just as 

 great if not greater variations than these. 



As the salinity of the water, or percentage of inorganic 

 salts present, has such a marked influence on the growth 

 of the larvae, it might be expected that they would react 

 similarly to changes in the organic constituents present. 

 Such proved to be the case. Thus larvae allowed to de- 

 velop in water in which other larvae had already undergone 

 development, and which therefore contained their products 

 of excretion, were diminished in size some 7*5 per. cent. 

 It was thought that this harmful effect might be partly due 

 to the presence of uric acid and urea excreted by the larvae, 

 but such was evidently not the case, as the addition of 

 small quantities of either of these salts to the water pro- 

 voked a considerable increase in the size of the larvae. 

 Thus by the introduction of one part in 70,000 of uric acid, 

 larvae 12 '2 per cent, larger than the normal were pro- 

 duced. With larger quantities of uric aicid the effect pro- 

 duced was less, till with one part in 28,000, larvae 2"i per 

 cent, smaller than the normal resulted. In water containing 

 about one in 60,000 of urea, the larvae were found to be 

 about 3 per cent, larger than those grown under normal 

 conditions. This result is very curious, and was quite un- 

 expected. No less curious are the results of the experi- 

 ments made on the effects of changes in the gases dissolved 

 in the water. Thus it was found that the amount of 

 oxygen gas dissolved in the water might be diminished 

 considerably without producing any effect, and, strange to 

 say, that the amount of carbonic acid present might be 

 increased by some 18 per cent.; to such a degree in fact 

 that the larvce would barely develop at all, and yet no 

 harmful effect on the size of the surviving larvae show itself. 

 It has perhaps been noticed that no reference has been 

 made to experiments on the effects of nutrition, this being 



