338 BIOLOGY 



Race Variations. In addition to individual variations, the 

 whole species may show a tendency to diverge from its original 

 form. Races are either slowly or rapidly changing from their 

 previous condition, so that if the members of any race living 

 to-day are carefully compared with those living in a previous 

 period of the world's history, it will be found that the whole 

 race has undergone a general change which has affected all 

 members. Such race variation commonly occurs by what 

 is known as divergence. By this is meant that the descendants 

 of one type have, by this race variation, diverged in several 

 directions, more or less different from each other. This is 

 explained by the assumption that the descendants from any 

 animal remain neither exactly like their ancestors nor like each 

 other, and that different lines of descent depart from the origi- 

 nal type in different directions. Examples of this are numerous, 

 but for illustrative purposes two well-known instances of such 

 divergence will be briefly mentioned. 



Breeds of pigeons. For some centuries, breeders of pigeons 

 have been very much interested in improving different strains 

 of these birds, and pigeon fanciers have been careful to breed 

 together individuals showing characters that appeal to their 

 fancy. The result has been that the pigeons have undergone 

 many profound changes from their original type. The original 

 pigeon, from which all of our domestic pigeons came, is fairly 

 well known to be essentially the same as the rock pigeon of 

 India, a bird gray-blue in color, with bars on its breast and a 

 tendency to perch on rocks, but never on trees. Historical and 

 scientific evidence shows that all the numerous strains of 

 pigeons with which our pigeon fanciers are familiar to-day 

 have been derived from this bird. The tumblers, the fan- 

 tails, the pouters, and hosts of others, have all been descended 

 from this primitive ancestral form. The differences between 

 these varieties are very numerous, including variations in 

 color, length of bill, size, wings, tails, and many other points, 



The differences between the breeds of pigeons, which have 



