20 



GENERAL PRINCIPLES OF ZOOLOGY. 



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

 comparison with the rest of the body (figs. lA, 1b); even the 

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



Fig. 1a.— English carrier-pigeon. (After Darwin.) 



Fig. 1b.— English tumbler-pigeon. (After Darwin.) 



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

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

 14 to 11. 



P>. Vuriidion within ilic Specica. — Now in respect to the occur- 

 rence of transitional forms and the constancy of differences, tliere 

 is witliin one and the same 'good species ' the greatest conceivable 

 difference. In many very variable siiecies the extremes are united 



