Shelford. — Physiological Characters and Depth 197 



V. — GENERAL DISCUSSION AND CONCLUSION. 



The resistance to fresh water and to high temperature 

 on the one hand and acid and alkaline water on the other 

 being reversed, we conclude the physiological characters 

 of the animals differ generally and that the differences 

 are not purely adaptive adjustments. Fluctuations in 

 temperature and salinity are greatest at the surface and 

 thus animals at the surface might be expected to show 

 greatest resistance to differences in these respects. On 

 the other hand since fluctuations in degree of alkalinity 

 are usually greatest in the region occupied by vegetation 

 it would accordingly be expected that animals from the 

 Laminarian Belt would be more resistant to alkaline con- 

 ditions than those from deper water but such is not the 

 case. 



The experiments indicate that it is not safe to assume 

 that individuals of a species have the same physiological 

 constitution regardless of conditions or that the presence 

 of a species coincident with a uniform condition of a given 

 factor such as temperature does not indicate that tem- 

 perature controls the distribution. The organism may 

 be physiologically different. 



In fresh water the presence of certain animals is often 

 taken to indicate that conditions are suitable or detri- 

 mental to fishes, or that the water is or is not contamin- 

 ated. Such conclusions must be made with due caution 

 and variations in physiological characters of such index 

 organisms must be fully investigated before their presence 

 can be relied upon to indicate the conditions they are 

 supposed to show. 



