26 GENERAL PRINCIPLES OF ZOOLOGY. 



enormous variations are found in the size of the beak and feet in 

 comparison with the rest of the body (figs. IA, IB); even the 

 skeleton itself participates in this variation, as is shown by the fact 



FIG. IA. English, carrier-pigeon. (After Darwin.) 



FIG. IB. English tumbler-pigeon. (After Darwin.) 



that the total number of vertebrae varies from 38 (in the carrier- 

 pigeon) to 43 (in the pouter), the number of sacral vertebra from 

 14 to 11. 



B. Variation within the Species. Now in respect to the occur- 

 rence of transitional forms and the constancy of differences, there 

 is within one and the same < good species ' the greatest conceivable 

 difference. In many very variable species the extremes are united 



