108 Atavism. 



appeared and threw a definite liglit on the probable na- 

 ture of tlieir 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- 

 qtiiry and in the highly important results which it has 

 afforded, is referred to tlie 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 wdiich underlie the reappearance of preexisting 

 c]iaracters. 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 whicli 

 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 



