20 



GENERAL PRINCIPLES OF ZOOLOGY 



similar to one another, two groups can be recognized which differ consider- 

 ably from each other, if the differences between them be obliterated by 

 no intermediate forms, and if in several successive generations they remain 

 constant, then the systematist speaks of a 'good species;' on the other 

 hand. he speaks of varieties of the same species when the differences are 

 slight and inconstant, and when they lose their importance through the 

 existence of intermediate forms. A definite application of this rule dis- 

 closes great incongruities, many groups being regarded by one set of 

 systematists as good species, by another only as 'sports,' i.e., as varieties 

 of the same species. The differences between the 'races' of our domestic 

 animals are often so considerable that formerly they were regarded not only 

 as sufficient for the foundation of good species, but even of genera and 

 families. In the fantail pigeon the number of tail-feathers, originally 

 only 12-14, has increased to 30-42 (fig. ic) ; among the other races of 





FIG. ic. English fantail pigeon (after Darwin). 



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 

 participates in this variation, as is shown by the fact that the total number 

 of vertebrae varies from 38 to 43, the number of sacral vertebrae from 

 14 to n. 



B. Variation within the Species. Now in respect to the occurrence 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 by many transitions; 



