108 Atavism. 
appeared and threw a definite light on the probable na- 
ture of their common ancestor. This is regarded as being 
an extinct form, with an hexamerous perigon of equal 
petals, and six stamens. A still living form, Iris falci- 
folia, possesses such a perigon but has only three stamens. 
The reader who is interested in this branch of in- 
quiry and in the highly important results which it has 
afforded, is referred to the works of this author for 
further information. 
I now return to the main question, viz., that of phys- 
iological atavism. Here we are concerned not with the 
production of new forms but with an inquiry into the 
processes which underlie the reappearance of preexisting 
characters. The character in question is, therefore, one 
that is still retained in that species from which the one 
under investigation is descended. Atavism is in this 
case to be regarded as an oscillation between two empir- 
ically known extremes. The field of oscillation can ob- 
viously not be very considerable, for only in cases of very 
close relationship is the common origin of two forms 
historically known to us. 
In this restricted province also, atavism may be 
brought about by fluctuating variation as well as by muta- 
tion. In the case of the former it is merely a transitory 
phenomenon and dependent on external conditions; but 
in the second case it leads to the origin of a race which 
externally resembles the ancestors of its parent form. 
Variational atavism seems to be a phenomenon which 
plays a large part in the sphere of semi-latent characters. 
As an example of this I cite the case, described above, 
of the five leaved clover ( 5, p. 36) which always bears 
a certain number of trifoliate leaves especially under 
unfavorable conditions. These trifoliate leaves obviously 
