CHAPTER VEE 
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 which 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. Lhe attainment of a functional condition by struc- 
tures normally suppressed, 
2. Reversion of organs and tissues to an original type. 
