CHAPTER VI. 



ATAVISM OR REVERSION. 



MUCH that is fanciful and speculative is mixed up with 

 the subject of atavism ; the widespread acceptance of 

 the principles of evolution has had the effect of render- 

 ing us less critical in the examination of suspected cases. 

 On the present occasion atavism or reversion will be 

 considered only in relation with structural peculiarities, 

 and an endeavour will be made to indicate spurious 

 examples of the process. 



Gegenbaur defines atavism as a " reappearance of a 

 more primitive organization, or a reversion to a primary 

 state." He is also careful to point out that atavism 

 does not consist in the existence of a latent germ, but 

 in its becoming perfected or further developed. This 

 view may be more clearly expressed thus : 



Atavism consists in the attainment of a functional or, 

 more or less ', full development of parts whicJi for a given 

 animal are suppressed during embryonic life, or undergo 

 great modification. 



This definition allows atavistic phenomena to be 

 arranged in two groups : 



1 . The attainment of a functional condition by struc- 

 tures normally suppressed. 



2. Reversion of organs and tissues to an original type. 



