MIGRATION AND ORIGINS 285 



acters. The versatility of specific physiological processes and be- 

 havior is made impressively apparent by this stability. Thus, al- 

 though it is often proposed that the occurrence of taxonomic races 

 shows an evolutionary process by which species have been formed, there 

 is no convincing evidence that the races of birds form steps toward the 

 evolutionary divergence of species. We can say that natural species 

 formation has not been evident during the enormous environmental 

 fluctuations of the last 100,000 years. It is amazing that the species 

 were physiologically so versatile as to meet these changes without es- 

 sential modification. 



