32 GENERAL PRINCIPLES OF ZOOLOGY. 



other races of pigeons 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 par- 

 ticipates in this variation, as is shown by the fact that the 

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

 pigeon) to 43 (in the pouter), the number of sacral ver- 

 tebrae from 14 to 11. 



FIG. ic. English Fantail Pigeon. (After Darwin.) 



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

 occurrence of transitional forms and the constancy of dif- 

 ferences, there is within one and the same " good species" 

 the greatest conceivable difference. In many strongly 

 varying species the extremes are united by many transi- 

 tions; in other cases sharply circumscribed groups of forms, 

 i.e. races, can be distinguished within the same species. 

 In the race, the peculiar characteristics are inherited from 

 generation to generation with the same constancy as in 

 good species. This is shown in the human races, and 

 many pure, cultivated races of domesticated animals. 



Physiological Characters. A. Crossing of Species 

 and Varieties. A critical examination leads to the con- 

 clusion that Morphology is indeed useful for grouping ani- 

 mals into species and varieties, but that it leaves us com- 



