THE TANGANYIKA PROBLEM. 
2 5 
gators that long before a poisonous close 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. LoebJ 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. 
f American Journ. Phis. Vol. iii., p. 385. 
| Archiv. Fur Entwic. Mechanik. Vol. xi., p. 617 — 1901. 
