EFFECT OF EXCRETION PRODUCTS OF PARAMAECIUM 559 



way due to the volume of water available for each snail, but he 

 was unable to determine how the volume produced the effects 

 noted. The same question was considered in 1894 by De 

 Varignyi° with considerably elaborated methods, and he found 

 that the size of the snails was affected by the number of individ- 

 uals in a given volume of water, but he did not observe that they 

 were so markedly sensitive to differences in volume of the water. 

 He concluded that the area of the surface exposed to the atmos- 

 phere had more influence than the volume. Snails were bred by 

 Whitfield^ 1 in a small aquarium for four generations and he found 

 that the size of the shell decreased during each succeeding gen- 

 eration and that various other morphological changes occurred. 



Yungi2 in 1885 made experiments with tadpoles and found that, 

 within limits, the larger the area of the exposed surface of the wa- 

 ter in which they were bred the more rapid their growth took 

 place. He concluded that the result was due to the water ab- 

 sorbing a larger proportion of oxygen from the air. 



An extensive series of experiments was made by Vernon, ^^ in 

 1895, in which, for example, he allowed eggs of several species of 

 sea-urchins to develop in water which had previously been the 

 environment for a considerable time of another batch of eggs, 

 and he found that in every case the larvae of the second batch 

 were diminished in size as compared with the control. He con- 

 cluded that products of metabolism excreted by the first batch 

 retarded the growth of the second. Other experiments apparently 

 showed that the growth of larvae was decreased by their own met- 

 abolic products, and that the products of excretion of adult echi- 

 noids acted more adversely both on the life and on the growth 

 of embryos if these belonged to the same species than if they 

 belonged to other species — in fact he actually found that the 



1° DeVarigny, Journ. de I'Anat. et de la Physiol., p. 147, 1894. Experimental 

 evolution, pp. 79-88, 1892. 



11 Whitfield, Bull. Amer. Museum Nat. Hist., vol. 1, p. 21. Amer. Naturalist 

 vol. 14, p. 51. . 



12 Yung, Arch, des Sci. Phys. et Nat. T. 14, p. 502, 1885. 



13 Vernon, Mittheilungen a.d. Zool. Sta. z. Neaple, 13, p. 389 et seq. Proc. Royal 

 Soc. London, vol. 186, B. p. 603, 1895. Variation in animals and plants, 1903. 



