C. L. PROSSER 349 



and many animals do function without it at restricted energy 

 levels. Also there are several alternate oxidative paths which can 

 bypass the cytochrome system. In other words, a genie change 

 interfering with single enzymes would not be readily detected 

 in complex animals, it would not likely become fixed because it 

 would be negative rather than positive or even neutral. However, 

 when enzyme changes have occurred in the direction of changes 

 in optima for a species, these have been selected; parasitic worms 

 can rely on anaerobic pathways, herbivorous insects tend to 

 have more active carbohydrases, and carnivorous ones more pro- 

 teases. However, there is no doubt that more sensitive functional 

 tests are needed. Also, it is not always easy to find the most crit- 

 ical or important functional characters. 



In summary, physiological adaptedness to the environment may 

 exist in the absence of clear morphological speciation (a) in 

 balanced polymorphic populations (these include advantageous 

 heterozygotes and genie expression differing with genetic en- 

 vironment), (b) in clines, (c) in sibling species, and (d) in the 

 extensive phenotypic lability in response of individuals to en- 

 vironmental stress. Physiological characters may not conform to 

 "good" specific characters because (a) taxonomic characters are 

 often nonadaptive, (b) physiological characters are complex 

 genetically ( multif actor ) , (c) adaptive characters usually have 

 large safety factors and parallel or alternate paths, and functional 

 characters are either (d) quantitatively so sensitive to the en- 

 vironment that phenotypic variation exceeds genotypic, or (e) 

 qualitatively so complex that reproductive isolation has already 

 been established. If evolution is considered as the development 

 of adapted organisms and if adaptation is basically functional, it 

 is of prime importance to analyze populations in terms of physio- 

 logical characters. However, the origin of species per se may not 

 coincide with the development of adapted organisms. 



Examples of Variation 



We may next consider examples of populations which have 

 been examined for physiological variation with respect to various 

 limiting environmental factors. We shall consider first for each 



