C. L. PROSSER 351 



marus zaddachi, which tolerates near-fresh water, and G. salinus, 

 which lives in the more saline regions of estuaries. The blood 

 concentration curves of the two forms have not been obtained, 

 but they are intersterile, and hence are effectively separate spe- 

 cies (Spooner, 1947; Kinne, 1954). 



Genetically distinct clones of Euplotes vassus differ in their 

 ability to withstand transfer to high salinity corresponding to 

 saline lakes. Populations of Euplotes in salt lakes appear to have 

 become selected for salt-resistance (Gause, 1947). By analogy 

 with varieties of Paramecium (see below) these strains of Eu- 

 plotes may well be natural species. 



The European stickleback, Gasterosteus aculeatus, occurs in 

 two subspecies (Heuts, 1947, 1949). One subspecies (gymnurus) 

 is predominantly a freshwater fish whereas the other (trachurus) 

 predominates in a brackish to marine habitat. The reproduction 

 (hatching) of gymnurus is better in fresh water, and that of 

 trachurus in salt water. Linked with salt-water tolerance is low 

 number of vertebrae, larger body size, and larger number of 

 lateral plates. Also the interaction between the effects of tem- 

 perature and salinity in affecting the number of dorsal fin rays 

 differs in the two forms. The physiological mechanisms of differ- 

 ences in salinity have not been examined, but this fish appears 

 to offer an example of the complex interaction of two environ- 

 mental stresses (salinity and temperature) and the linkage with 

 gross morphological characters. The two subspecies differ in many 

 ways, and despite the fact that they produce fertile hybrids, 

 natural selection favors the two extremes; they appear to be well 

 on the way to becoming natural species. 



A similar example, now recognized as sibling species, is the 

 freshwater Anopheles gambiae and the morphologically similar 

 A. melas which normally breeds in brackish water and can 

 develop in 150% sea water. Hybrids are sterile, and the eggs 

 are morphologically distinguishable (Ribbands, 1944). The trop- 

 ical forest mosquitoes, Anopheles bellator and A. homunculus, oc- 

 cupy canopy levels which overlap on one side of each range; 

 bellator rises in the forest heights in the evening and descends 

 less far in the day because of its greater tolerance of low hu- 



