RELATIVE STABILITY OF LIVING MATTER 305 



evolution of the world as a whole. Existing species therefore 

 represent the choicest material of the organic world. Having 

 arrived at a high state of differentiation, they have doubtless 

 lost something of the flexibility that marked their early and more 

 generalized forms, yet they are to be regarded as " highly selected 

 material," ready to the breeder's hand for still greater adapta- 

 tion, not only to their own needs but to those of man. 



Even when dealing with specially flexible forms, the breeder 

 is never to forget that they are constantly giving rise to branches 

 that are incapable of adaptation. Unhappy is the breeder who 

 devotes his life to a branch of this kind, whether it be among 

 horses, cattle, or any of the more slowly multiplying forms, animal 

 or plant ; for no amount of apparent variability will make up for 

 inherent defects, nor will it, seemingly, avert the evil day of 

 extinction. 



SECTION III EVIDENCE FROM REVERSION AND 

 ATAVISM i 



The sudden reappearance of a long-lost character serves to 

 remind us that combinations once effected tend strongly to 

 return. The English breeds of cattle are supposed to have 

 descended from the ancient wild white cattle that roamed freely 

 over the island until the year 1 200 or later, when private owner- 

 ship interfered. With the inclosing of the larger estates as hunt- 

 ing parks, herds of these cattle were included with other game 

 animals, and for over six hundred years they have lingered in this 

 semi-wild condition at Chillingham, Chartley, and other parks. 



1 It is important that the student observe the modern distinction between 

 " reversion " and " atavism." They both refer to the reappearance of characters 

 once typical but now extinct. " Reversion " is the term to use when the character 

 belonged to a near-by ancestor clearly of the same species but several generations 

 removed, while " atavism " is used to denote the appearance of characters belonging 

 to exceedingly remote ancestors, perhaps even of different species. An example 

 of reversion is the occasional appearance of the white color in the red breeds of 

 English cattle, and a good example of atavism is the occasional persistence of 

 gill slits in mammals, which generally disappear during embryological development 

 but occasionally remain as permanent openings. Canine teeth in man, normally 

 present, are regarded as a heritage from some primitive ancestor. They may some 

 day become atavistic. The term " reversion " covers most of the breeder's needs. 

 It is the biologist who deals with remote species. 



