C. L. PROSSER 357 



In Paramecium aurelia there are eight varieties which are 

 really natural species (Sonneborn, 1950); two of these are dis- 

 tinguished by the temperatures optimal for their growth and re- 

 production ( Sonneborn and Dippell, 1943 ) . 



Oxygen. A considerable amount of nongenetic variation with 

 respect to oxygen requirement is possible. For example, intestinal 

 parasites and such free-living animals as Tubijex and Daphnia 

 are able to alter their metabolism or their oxygen transport sys- 

 tem according to the environmental oxygen. It is entirely possible 

 that some of the effects on freshwater poikilotherms attributed to 

 temperature are in fact effects of oxygen, since in winter the 

 oxygen in stagnant ponds where there is no photosynthesis may 

 be very low. Altitudinal effects attributable to oxygen lack have 

 been reported only for birds and mammals where the phenotypic 

 response of increase in blood hemoglobin is well known. Of two 

 species of a Russian mouse, Apodemus, one, sylvaticus, lives in 

 both mountains and plains, the mountain populations having 

 higher hemoglobin concentrations, while the other, agrarius, is 

 restricted to the plains. Apparently at high altitudes members of 

 the latter species fail to acclimate by increasing their blood 

 hemoglobin (Kalabuchov, 1937). 



Oxygen gradients do not in general provide good geographic 

 series, but microecologically oxygen supply may be important in 

 the isolation of populations. For example, in a series of chirono- 

 mid larvae, the stream species survived anoxia only a short time 

 whereas species from low-oxygen ditches survived anoxia for 

 about 100 hours (Walshe, 1948). Gammarids and planaria from 

 fast well-aerated water have higher rates of oxygen consumption 

 than do related species from sluggish water poor in oxygen. Some 

 snails and numerous parasitic helminths get along well on the 

 energy of glycolysis and excrete the acids which are formed. In 

 fact, several patterns of repayment of oxygen debt are recognized 

 (von Brand, 1952). The known genotypic variations recognized 

 with respect to oxygen are largely at the species level. 



Foods and Toxins. There are marked interspecific differences 

 in digestive enzymes according to natural diets and a few in- 

 stances of slight differences in amino acid requirements, but in 



