THE TANGANYIKA PROBLEM. 25 



gators that long before a poisonous dose is reached the 

 addition of chemical substances to water in which animals 

 are normally living acts as a stimulus towards variation 

 either in themselves or in their offspring. Thus Vernon* 

 found that the addition of small quantities of urea and uric 

 acid, ammonium chloride, and nitrates had an appreciable 

 effect on the larvae of sea-urchins which were reared in such 

 solutions. Herbstf showed that lithium salts produced very 

 abnormal effects upon the growth of larvae. Loeb^ also 

 showed that change of salinity affected the growth of tubu- 

 larians. It would thus appear that there is some reason to 

 believe that a gradual increase in the salts normally contained 

 in the water of the sea would produce effects analogous to 

 those which unquestionably have taken place in the animals 

 living in it during past times. Such an increase would, it 

 appears, ultimately kill off some animals, weaken other 

 stocks, cause what we may call rampant variation, and force 

 a number of forms into the fresh-waters of the globe. 

 From what has been observed there is also reason to 

 believe that, had such an increase of the salinity of the sea 

 occurred, it would as a matter of fact begin to affect large 

 numbers of diverse organisms about the same time, or, in 

 other words, the same percentage of salts would begin to 

 tell on a large number of diverse types. If, then, the sea 

 has become increasingly salt, this simple cause would be in 

 itself efficient to account for a number of the most perplex- 

 ing features which the universal fresh-water fauna of to-day 

 presents. 



The study of the general nature of fresh-water faunae 



* Phil. Trans. Roy. Soc. Vol. i86b., p. 577—1895; also Naples. Mitthal, 

 Vol. xiii., p. 341 — 1898. 



j American Journ. Phis. Vol. iii., p. 385. 



\ Archiv. Fur Entwic. Mechanik. Vol. xi., p. 617 — 1901. 



