ADAPTIVE VAEIATIONS. 385 



surface, and so are frequently uncovered. They must 

 therefore be exposed to sea-water diluted by rivers, and 

 to rain water. The less resistant Anthea is found fur- 

 ther below the surface, however, whereas the still 

 less resistant Sagartia lives in water several metres 

 deep. 



Observations on acclimatisation to saline solutions 

 are, perhaps, less important and less interesting than 

 those on acclimatisation to other conditions, in that, 

 within certain limits, the phenomenon is probably a 

 purely physical one, dependent on differences of osmo- 

 sis, and the pressures and strains thereby set up. 

 There is little doubt that if sufficient care and time be 

 employed, any marine organism could be acclimatised 

 to fresh water, and any fresh water form to salt water, 

 or solutions of even greater density. If it be remem- 

 bered that the osmotic pressure of a 1 per cent, solution 

 of sodium chloride is over seven atmospheres, then it is 

 obvious that the strain upon the tissues of an organism 

 suddenly transferred from one solution to another of 

 considerably greater or less salinity may easily be suffi- 

 cient to rupture and kill them. 



Direct observations on the acclimatisation of the 

 vertebrata are extremely few, except in the case of cer- 

 tain mammals experimented on in connection with 

 serum therapeutics. Davenport and Castle * have 

 made some interesting observations on the acclimatisa- 

 tion of tadpoles to heat, however. Recently laid eggs 

 of Bufo lentiginosus were divided into two lots, one of 

 which was allowed to develop in a warm oven at a tem- 

 perature of 24 to 25, and the other kept at 15. 



* Loc. cit. 



