THE VIEWPOINT OF A PHYSIOLOGIST 



and the cline as a whole may show progressive functional adapt- 

 edness. Usually sibling or morphologically similar species are 

 first considered as physiological races, but as they are studied 

 more intensively, they turn out to be natural species. Careful 

 observation usually reveals minor neutral morphological differ- 

 ences by which sibling species may be distinguished. 



5. Physiological characters tend to be more sensitive to the 

 environment in their expression than do structural ones, and 

 the potential lability of individual animals is very great. The 

 changes which can be brought about by acclimation are extensive, 

 and as more populations are subjected to acclimation tests, more 

 and more do their differences turn out to be nongenetic. Many 

 so-called races are considered such merely because of their 

 ecology and they have not been subjected to adequate acclima- 

 tion tests. 



6. Most physiological characters which have been analyzed in 

 animals are polygenic. In fact, a well-adapted animal is suited 

 to its entire environment, not merely to a single physical factor, 

 and it is adapted not through the action of one gene but by its 

 whole integrated genotype. A gene will often be expressed differ- 

 ently if the accompanying genes are changed. On this basis it is 

 perhaps futile to expect adaptive physiological characters to be 

 as susceptible of analysis as distinctive morphological ones. 



7. It may be argued that the physiological criteria which 

 have been applied arc too gross, that measurement of oxygen 

 consumption by an animal is no more precise than stating the 

 number of segments on an appendage, that what is needed arc 

 more sensitive tests, measures of single enzymes. However, the 

 one gene-one enzyme hypothesis as developed so beautifully for 

 Neurospora may be ol limited applicability since it concerns 

 only defects in ability to synthesize relatively small molecules. 

 Very rarely is any animal with complex dietary requirements 

 dependent on any single enzyme pathway for either a specific 

 synthesis or degradation. There are numerous alternate pathways 

 l>\ which energy can be obtained from basic substrates. It is true 

 that interference with cytochrome oxidase 1 would be disastrous 

 lor most animals, yet this enzyme must be under multiple control, 



