462 



THE GENERAL PRINCIPLES OF ZOOLOGY 



formerly scientists recognized them, not only as distinct species, 

 but in some cases as belonging to different genera even 

 (Fig. 419); the fantail has from thirty to forty-two tail quills, 

 while other pigeons have only twelve or fourteen ; the carrier 

 has thirty-eight vertebras, the pouter has forty-three. These 

 differences and many others are most fundamental; but Darwin 

 succeeded in showing, by careful crossing, that all these different 

 types would revert to a type very similar to the wild rock pigeon, 

 Colnmba livia, which must have been the ancestor of them all, 

 and of which they are consequently merely so many varieties. 

 Thus he demonstrated that species are not fixed, but that varie- 



FlG. 420. Phyllopteiyx ?,;tn's, an Australian sea horse closely resembling in form and color 

 the algae amongst which it lives. (From Ludwig-Leunis" Synopsis der Thierkunde.) 



ties are in reality incipient species, — species in the process of 

 development, and in consequence species are incipient genera. 



Now Darwin has undertaken to show that nature exercises a 

 power of selection over wild animals similar to that exercised by 

 man over the domestic animals, but acting much more slowly. 

 He pointed out that many more plants and animals come into 

 existence every 'year than can possibly find room and food 

 enough on this earth. Many young animals are devoured by 

 other animals, many are killed by storms or other unfavorable 

 conditions, and even as the animal approaches adult life it still 

 has many enemies. There is, then, a continual struggle for 

 existence on the part of every animal and plant ; plants are 

 preyed upon by animals and in turn prey upon one another, - 



