392 
Washburn (’15) in my chapter in Ward and Whipple’s “Fresh Water 
Biology” are erroneous. Shull’s “Principles of Animal Biology” (page 
277) includes statements of a similar nature which should be corrected. 
These various statements relative to CO, optima, etc., are not to be re- 
garded as errors when made; later evidence has merely shown that hy- 
drogen ion concentration is a better index than CO,. 
There are a number of general reasons for determining hydrogen 
ions, such as the close regulation of their concentration in the animal 
body and their apparent great physiological importance in vertebrates. 
The writer does not, however, wish to give the impression that he means to 
advocate the use of pH determinations to displace any other determina- 
tions now in common use unless it be CO,, and not even in this case until 
further investigations have been conducted. 
CoNnCLUSIONS 
Hydrogen ions should be determined for the following specific 
reasons: 
1. The effect of various concentrations of oxygen are modified by 
the hydrogen ions present, particularly the ill effect of low oxygen on 
survival and development, which is increased by the high hydrogen ion 
concentration which accompanies it in waters of low alkalinity. In a 
general way the hydrogen ion concentration varies inversely with the 
oxygen content in bodies of water with stable alkalinity (e. g., see Tables 
VI and VII and Graph 1). 
2. With the same oxygen content, different hydrogen ion concen- 
trations have definite effects on the rate of development and time of 
survival of aquatic animals. 
3. Many animals react definitely to hydrogen ion concentration 
(fishes, crayfishes, Entomostraca) in a manner similar to their reactions 
to temperature. Each species tolerates a rather wide range with a 
fairly definite optimum. 
4. The determination of hydrogen ions is to be preferred to CO, 
determinations because it is the free hydrogen ions which are most effec- 
tive, and, as shown in Table IX, column 6, for example, while the CO, — 
remains constant (6 p.p.m.), hydrogen ions concentration changes by 
tenfold (pH 6.6 to 7.63) owing to difference in alkalinity. 
5. Colorimetric determinations of pH are about as accurate as CO, 
determinations and can be made on the spot very quickly. With some 
simple device for collecting (see Richardson’s Fig 6, page 372, Vol. 
XIII of this series), aeration can be reduced, and very small quantities 
of water used as contrasted with the usual 250 c.c. samples. 3 
6. The distribution of plants and animals is correlated with hydro- 
gen ion concentration. 
